SSH version 3 exists already (known as SSH G3 since 2005)

[vam67423]

The original inventor of the SSH protocol, SSH Communications Security has already published version 3 of the SSH protocol in 2005, also known as SSH G3. This version was also requested as a new standard with the IETF which would likely prevent the reuse of a similar name like SSH3.

Source:
https://www.helpnetsecurity.com/2005/02/14/ssh-unveils-ssh-g3-a-powerful-third-generation-secure-shell-architecture-for-ssh-tectia/

[costinm]

Also changing the transport should not be a new protocol version - for example RFC 8484 defines "DNS over HTTPS" not DNS3. I would expect similar SSH over WebTransport or SSH over HTTP2 projects to not use SSH4, SSH5 (i.e. versions of the core protocol), but use names that are specific to the change.