See https://www.wikidata.org/wiki/Special:Tags on Wikidata and sort it by the third column. The first, second, third and fourth tag have the third column empty but their MediaWiki: pages do exist. (Best viewed when you as sysop and can see "(edit)" links.) I think it's because the tags include '/' symbol.
Version: 1.23.0
Severity: normal
Yeah, slashes in tag names (and thus in names of MediaWiki: namespace pages) are probably impossible to support due to how our internationalization system works.
I'm not sure what we could do about it, maybe map '/' to '-' before looking for a message or something? (So the page for a tag called "a/b/c" would be "MediaWiki:A-b-c".)
I think the solution here is to prohibit forward slashes in change tags. This is planned to be enforced for manual tag creations in https://gerrit.wikimedia.org/r/181958 but it's not clear how that can be enforced for extensions. Well-behaved extensions would ideally call ChangeTags::canCreateTag (introduced in that patchset) to see if a tag name is valid, which would reject names like adding potentially mistaken label/description/alias.
As for the local issue on Wikidata, I think the answer to that is to coordinate between a Wikidata admin and shell user, so that the admin renames the tags to something else in AbuseFilter, and immediately afterwards, the shell user runs an UPDATE query on change_tag.