It's been in the air and mostly in sparse @JonRowe's comments (1, 2). This is an attempt to gather the ideas and start the discussion.

Release step-by-step plan

pre-4.0:

• fix internal deprecation matchers Internal affairs: expect_warn_deprecation matches any message when there's fail in the example rspec-support#451
• add deprecations for what we end up removing in 4.0
• • implicit block syntax (Print deprecation message when implicit block expectation syntax is used rspec-expectations#1139, Prevent should from circumventing target check rspec-core#2800)
• • Add deprecation warnings for features to be removed/changed in RSpec 4 rspec-core#2880
• • should (monkey and non-monkey), syntax config options
• • globally-exposed DSL, and its config option
• • shared_context_metadata_behavior (discussion in Reconsider shared_context_metadata_behavior rspec-core#2832)
• • run_all_when_everything_filtered
• • ... a couple others
• • Add deprecation warnings for features to be removed/changed in RSpec 4 rspec-mocks#1418
• • should Mocks syntax
• • allow_message_expectations_on_nil example method
• • Add deprecation warnings for features to be removed/changed in RSpec 4 rspec-expectations#1301
• • operator matching (is_expected.to be is a replacement for should, not just is_expected.to)
• • legacy matcher support
• • include_chain_clauses_in_custom_matcher_descriptions
• • present tense predicates
• • [ ] deprecate currently_executing_a_context_hook and recommend using current_scope instead (Add RSpec.current_scope method to replace currently_executing_a_context_hook? and self.inspect hack rspec-core#2895)
• • [ ] ? AnyInstance
• • [ ] ? DRb
• • [rspec-core] RSpec 4 deprecation warnings #106
• • [rspec-expectations] RSpec 4 deprecation warnings #124
• • [rspec-mocks] RSpec 4 deprecation warnings #117
• • Warn when include_examples is used with a block #243
• • [rspec-core] Add deprecate warning when example doc string is not a string object #107
• • config.warnings= boolean setting deprecation #268

4.0.0:

• Drop support for EOL Ruby versions 1.8.7 - 2.2 (Drop Ruby < 2.3 support rspec-core#2787 Drop Ruby < 2.3 support rspec-support#436 Drop Ruby < 2.3 support rspec-mocks#1349 Drop Ruby < 2.3 support rspec-expectations#1231)
• remove monkey-patching (globally exposed DSL and .should) (Scrape out monkey patching rspec-core#2803 et al., all interlinked)
• Remove deprecated features
• • implicit block expectation syntax (Restrict block matchers use with value expectations rspec-expectations#1285)
• • run_all_when_everything_filtered (Remove run_all_when_everything_filtered configuration option rspec-core#2845)
• • shared_context_metadata_behavior (Remove shared_context_metadata_behavior config option and make apply_to_host_groups the default rspec-core#2834)
• • include_chain_clauses_in_custom_matcher_descriptions (Remove include_chain_clauses_in_custom_matcher_descriptions option rspec-expectations#1279)
• • legacy matcher support (Remove support for legacy RSpec matchers (pre 3) rspec-expectations#1253)
• • raise where we previously warned (Raise instead of warn on long-deprecated usages rspec-core#2849)
• • present-tense dynamic predicates (Remove support for present tense predicates rspec-expectations#1286)
• • Remove deprecated :example_group example metadata sub-hash (Remove deprecated :example_group example metadata sub-hash rspec-core#2851)
• • Remove deprecated Hash-like behavior from example execution result (Remove deprecated Hash-like behavior from example execution result rspec-core#2862)
• • Various Core deprecations (Remove deprecations rspec-core#2864)
• • Remove deprecated allow_message_expectations_on_nil example method (Remove deprecated allow_message_expectations_on_nil example method rspec-mocks#1410)
• • Remove match_regex matcher and StartAndEndWith matcher base class from Expectations (Remove a couple of remaining deprecations rspec-expectations#1288)
• • [ ] Remove currently_executing_a_context_hook? (Remove deprecated currently_executing_a_context_hook? rspec-core#2911)
• set new defaults
• • strict_predicate_matchers (Turn strict_predicate_matchers on by default rspec-expectations#1277)
• • verify_partial_doubles = true (Verify partial doubles by default rspec-mocks#1409)
• [ ] Remove DRb runner (Remove DRb client runner rspec-core#2866)
• extensively test (our current projects, repos from https://github.com/pirj/real-world-rspec)
• unify multi-condition filtering (ticket Unify multiple-condition filtering rspec-core#1821)
• add a warning when doc string is not a string (Add a warning when doc string is not a string rspec-core#2922)
• restrict passing a block to include_examples/include_context (Raise on include_examples/context with a block #259)
• remove deprecated warn_about_potential_false_positives ([rspec-expectations] Remove deprecated warn_about_potential_false_positives #260, Updates for on potential false positives #199 )
• Raise on include_examples/include_context with a block Raise on include_examples/context with a block #259
• Raise when docstring's are not a string (or nil) Raise ArgumentError when docstring's are not a string (or nil) #252
• Change config.warnings= to accept :all/:deprecations_only/:none for Ruby warnings Set :all/:deprecation_only/:none as options for config.warnings= #161

post-4.0

• update RubyGems [new gem generator [1, 2]
• update rspec-rails/rspec-collection_matchers/rspec-its to support RSpec 4
• extract monkey-patching .should to a gem
• kwargs delegation
• [?] update transpec to facilitate migration from 3.x to 4.x (describe - > RSpec.describe?)
• Cleanup: CI scripts
• --no-document for gem
• unindent/dedent -> <<~
• rearrange gemfile/gemspec (runtime dependencies to gemspec, development - to gemfiles)
• figure out why rubocop passes while there are offences in lib (core, probably others too)
• set TargetRubyVersion for RuboCop, and bump to latest (that supports parsing 2.4, which even the edge should)
• tighten and improve minimum_coverage
• update rake, cucumber, aruba to latest versions (cucumber to 4.1 because 5.x depends on Ruby 2.5+, or even 3.x because of diff-lcs)

Ruby support

TL;DR since time has passed, we decided to drop Ruby 2.x support altogether in RSpec 4

------ Outdated 2021 research below

EOL Ruby means we support 2.5+, since 2.4 is EOL in April 2020
However, according to 2020 RoR community survey

70% use 2.5+
81% use 2.4+
89% use 2.3+

Do 2.3 and 2.4 cause any trouble? I could only find this:

        # 2.4 has improved BigDecimal formatting so we do not need
        # to provide our own.
        # https://github.com/ruby/bigdecimal/pull/42
        classes.delete(BigDecimalInspector) if RUBY_VERSION >= '2.4'

I can only think of RuboCop's TargetRubyVersion setting that has a minimum of 2.4. That means we won't be able to run rubocop on 2.3 CI build.

More on this in #25

Remove deprecated features/Introduce new defaults

• shared_context_metadata_behavior
• monkey-patching mode
• should syntax (with implicit receiver only, where should can be backed by is_expected.to. extract to a gem?)
• implicit block syntax (deprecated in Print deprecation message when implicit block expectation syntax is used rspec-expectations#1139)

Consider deprecating long-hanging/obscure features

• [ ] DRb runner. I only know of Spork that was using it, but its 2.0.0-rc is years old now with no signs of life

Cleanup

• rspec-core: "Ruby 1.9 has a bug that can lead to infinite recursion and a"
  Drafted in commits: ce2f4a0b 123e6ee7
  Drop Ruby < 2.3 support rspec-core#2787 (review)

  RSpec::Matchers.define(:be_one) do
    match { |actual| actual == 1 }
  end
  RSpec.describe do
    alias_method :b, :be_one
    it { expect(1).to be_one }
  end

• for all repos

  • bump cucumber to ~> 5.2.0, cucumber-ruby (undefined method with_filtered_backtrace') + fix deprec titletoname`
    • 1.3-2.0 don't work for me locally with a weird error: undefined method []' for nil:NilClass (NoMethodError) /Users/pirj/.rvm/gems/ruby-2.7.1@rspec-core/gems/cucumber-1.3.20/lib/cucumber/core_ext/proc.rb:17:in file_colon_line'
    • 2.4.0 spits "undefined method `with_filtered_backtrace'"
    • 3.2.0 - "undefined method 'ok?'"
    • 4.1 depends on diff-lcs (< 1.4, >= 1.3, ~> 1.3), while we depend on = 1.4.4
    • 5.2.0 Found tags option '~@wip'. '~@tag' is no longer supported, use 'not @tag' instead
  • update diff-lcs ">= 1.4.4" (Stop using DiffHelpers rspec-support#477 and co)
  • update rake to latest
  • skip documentation
    yes | gem update --system --no-document
    yes | gem install bundler --no-document
    also on sub-builds?
  • rearrange Gemfile/gemspec
    e.g. core: simplecov, test-unit, aruba, rake
    To my best knowledge, add_development_dependency only affects local bundling. I've moved it over from the Gemfile.
    As far as I understand we can shuffle those dependencies between add_development_dependency and Gemfile without affecting anything.
    To me, it's better to keep those gems that we directly depend on here, while reserving the Gemfile to add version constraints for third-party dependencies like ffi and those that require a conditional, e.g.
    if RUBY_VERSION >= '2.4'
  • figure out why rubocop lib passes on core
    frozen string literal - run with --only Style/FrozenStringLiteralComment, why it doesn't detect offences without --only?
    lib/rspec/core/backport_random.rb excluded
  • <<- chomp gsub unindent dedent -> <<~
    • remove write_file_formatted in rspec-core

kwargs delegation

• 💚 Regression in rspec-expectations 3.9.3 if argument to predicate method implements to_hash rspec-expectations#1221 (comment)
• ⚠️ Keyword args support causes expected values to coerce to Hash rspec-expectations#1241 (comment)
• https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/#a-compatible-delegation
• https://eregon.me/blog/2019/11/10/the-delegation-challenge-of-ruby27.html
• https://github.com/rspec/rspec-expectations/pull/1222/files#r513326762
• Compatibility of shared_examples with Ruby 3 rspec-core#2674
• https://github.com/rspec/rspec-core/pull/2716/files
• rspec/rspec-expectations@964196a