If I am reading the intent of T292552: Rename articles and users to update our case mapping to PHP 7.4 and Unicode 11 right, PHP will and must instead go the other way.

In T292824#7412307, @Izno wrote:

If I am reading the intent of T292552: Rename articles and users to update our case mapping to PHP 7.4 and Unicode 11 right, PHP will and must instead go the other way.

No, we'll have a permanent override for Georgian and the eszett character. We're not going to completely break kawiki just because Unicode told us to. {{ucfirst:}} is an attempt to implement title case, and as such, shouldn't map Georgian Mtavruli and probably shouldn't map Eszett. The other ligatures (apart from eszett) are also broken, e.g. fi should really become Fi not FI.

The ucfirst in mediawiki.jqueryMsg should use the upper case function from mediawiki.Title which already uses the same capitalization like in PHP

Sharing code with mediawiki.Title sounds like a good idea to me. The PHP override list is configurable, which is awkward, but whatever the solution ends up being, it will be easier with a single JS implementation.