Per https://phabricator.wikimedia.org/T298715#7609769.
You should implement messages "MediaWiki:Mainpage-title" and "MediaWiki:Mainpage-title-loggedin" in skins Modern and CologneBlue, as it already has been done for skins Vector and Minerva, and expected to be in progress for skins Monobook and Timeless.
These skins Modern and CologneBlue are no longer listed in the preferences, though they can be tried by using useskin=modern or usekin=cologneblue in the query string.
The message has been implemented in Modern: https://github.com/wikimedia/mediawiki-skins-Modern/commit/35c9337d207714c75716bfb5f1b6ca11c4d2f5fb.
So Cologneblue is the only remaining skin that doesn't implement the message, yet.
(by the way, refs https://phabricator.wikimedia.org/T223824 and https://phabricator.wikimedia.org/T287616)
Untagging Modern as it's fixed like you say.
FYI Both Cologneblue or Modern are only maintained from a technical perspective so they don't cause fatals in productions. There should be no expectation that either of these skins work for anything out of the box (which is why they are hidden to users in preferences).
For the record, the message has been implemented in CologneBlue too: https://gerrit.wikimedia.org/r/776234.