This is certainly The Buf Language Specification, which is useful in itself. Specs are like contracts. If I were to build a tool to be compatible with Buf, I would definitely aim it to work with this spec.
The problem is that the Protobuf project simply didn't sign this contract. Whatever it says is, sorry for the choice of word, a bit pointless if I'm trying to build a tool compatible with Protobuf, specially around forward compatibility.
The industry does need better tooling around protobuf/efficient RPC, and being dependent on a single company (i.e. Google) is definitely not healthy. I hope you guys succeed in what your are trying to doing.