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.