> Once these middlebox vendors catch up to TLS 1.3, what's to stop them from breaking the supported versions extension the same way they broke the client versio

> Once these middlebox vendors catch up to TLS 1.3, what's to stop them from breaking the supported versions extension the same way they broke the client version field Nothing. It will happen just the same, so TLS 1.4 will have a “actually, this is the extension to use to determine the version”-extension