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:

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