For example, google talk originally used xmpp. They kept adding features that broke on the xmpp side of things, until people effectively used google talk. They then cut of xmpp, after successfully killing it.
lemmy.ml is the instance run by the lemmy developers, who are tankies. Tankies are people who support authoritarian regeims such as the soviet union, or north korea.
As such, they sometimes remove anti-china or pro-ukraine posts/comments.
So, the no matter what instance thing isn't really true. Moderation is different per instance, mods can remove comments locally.
Account migration is planned, but not there yet.
https://lemm.ee/post/57060912