Protocol handshakes to establish versions is the easiest way and has been used for ages; every https connection you start to read this message performs a handshake.

Software implementations may be strict or non-strict about versions supported. It's not hard; just needs someone with experience to champion it.

optimism

nostr

Mastodon Exit Interview (with Bluesky entrance interview)

k00b

It does need a v2, but I think it's either very close to (or already past) the point where a v2 can be implemented without both killing the V1 and dooming the V2 to failure. If there is still time I also don't know how a v2 (several breaking changes) gets done, besides someone influential simply forking it and successfully marketing their fork. 