As you may have been aware, we've been working on changing how MediaWiki stores comments: instead of having them as fields in each revision (rev_comment), log entry (log_comment), and so on, we're storing the text in a central "comment" table and referring to them by ID from other tables (log_comment_id and so on).

We've been writing to the new fields and tables since the end of February 2018, and have back-populated them for old revisions, log entries, and so on. Now we're starting to look at stopping the writes to the old fields, starting soon with testing wikis such as test.wikipedia.org and also with mediawiki.org. Other wikis will follow, likely in October after the DC switch-back.

For the most part wiki users shouldn't notice any changes, however if you notice something not displaying a comment or edit summary for new entries, on these wikis let me know.

For users of the Data Services replicas, such as Toolforge, the views currently simulate the old columns for you. However this may change in the future. See https://phabricator.wikimedia.org/T181650#4581384 for details.

MediaWiki developers should make sure code accessing comment fields makes use of the CommentStore class that was introduced in MediaWiki 1.30.

You can watch https://phabricator.wikimedia.org/T166733 (and any subtasks) for more information on the deployment process.