Well, do you think its relevant to support anything else other than Git nowadays?

Yes. There are people who prefer Mercurial than Git, and there are quite a lot of projects out there which still use Subversion.

I really don’t understand the mindset that leads to a belief that a monoculture is good or desirable.

Allura might do a bit more on that (…)

Yes, it does.

(…) but Gitea has way more features and the UI is actually decent.

That’s debatable, but to each its own.

Perhaps this could shine some light on the "do you think it’s relevant to suport anything else other than ".