After reading bug 47872 comment 10 and looking for usage of mw.log.deprecate at
https://github.com/wikimedia/mediawiki-core/search?q=deprecate%28&type=Code
I believe the deprecation of "mwCustomEditButtons" is still to be done.
Creating this bug for that.
Version: unspecified
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=47872