The feature request includes the phrase "Behind the scenes there would need to be some cryptographic magic," because it's just yet another "I have a sweet idea for an app, bro, I just need you to do the coding for me" pitch.
The project has 766 contributors with pull requests being accepted all the time. That's not a walled garden.
What I meant by "walled garden" is about federation and not commits, like look at the Signal app federation situation, they rejeted it because it might be confused with cyptocurrency, which I did hate because of its volatility
Alternative idea .... Someone uses OpenConext to provide a federation hub connecting minetest servers with identity providers. You can even call it LuantiID.
I don't see how this is such a big issue. Would feel weird to not be able to change username per server. Sometimes as my name for playing with friends or a pseudonyme if playing with strangers?