MediaWiki has an experimental configuration flag $wgUseCategoryBrowser that
alters the appearance of categories. If you don't use this flag or are not
aware of it you can safely ignore this message.
We plan to remove this configuration flag and associated code in MediaWiki
1.38 without deprecation warnings given its experimental nature:
https://phabricator.wikimedia.org/T298553
If needed an extension can provide the same functionality, so if anyone
needs this functionality please let us know by replying to this email or
the Phabricator ticket to help guide us better.
Thanks for reading
Jon