The image_comment_temp table was created to temporarily hold the new fields needed for T166732: Refactor comment storage in the database and abstract access in MediaWiki so it wouldn't have to be held up waiting for an expensive schema change on Commons' image table. The plan was always to eventually merge it into the image table once we had time to do that schema change.
Thanks to T187089, that image schema change is happening sooner than we had thought.
The rough plan here is:
- Write (and merge) the patch to add the img_descrption_id field to the image table, and the maintenance code to migrate data from image_comment_temp to image, and the changes to CommentStore to
- Actually perform the schema change on WMF sites (already in progress, see T187089)
- Configuration changes to start writing img_description_id.
- Run the maintenance.
- Write (and merge) the patch to drop the image_comment_temp table and the code in CommentStore for using it.