Just waiting for Minecraft to be so reverse engineered as to be its own protocol with multiple server and client implementations that just work.
it already is, the entire protocol is reverse engineered, there are tools to automatically deobfuscate the code and there is already a full reimplementation of minecraft that also supports servers
Does that mean there are open source clients that can connect to servers without a microsoft account?
the client has to authenticate with a central server and present a ticket to the server it wants to connect to. otherwise clients could impersonate each other easily.