I've seen that draft. It's not backwards compatible in any ways that v6 isn't.

The people designing v6 knew from the start that interoperability with v4 was a requirement. It's written right into the list of requirements for v6[1], right in the same place that they say that a flag day is never going to happen.

Why do you believe otherwise? The RFC is right there for anybody, including you, to read.

[1] https://datatracker.ietf.org/doc/html/rfc1726#section-5.5