Not directly, but if the most used implementation is unmaintained how popular is the language?

These days? Not very. However a lot of systems created in the 2000s, particularly enterprise software because XML was seen as the thing for enterprises, depends on it.

It’s not (and never was outside of corporate webapps) very common on the web, but there are still legacy things that need it.

Its a niche domain specific programming language. Its fairly popular in its niche (i.e. transforming xml documents to other formats), but that niche is kind of dying as xml wanes in popularity.

There are definitely still users, although a lot of them are probably outside the browser.

I did some work with XSLT back when it was enjoying some popularity (i.e. > 20 years ago.)

While I understand the appeal of the concepts behind XSLT, a language like that being expressed in XML is just... unfathomably perverse.

It's a positive testament to the industry's taste that XSLT essentially died.