https://cr.yp.to/djbdns/ipv6mess.html
still as relevant as the day it was written
replies(3):
That’s like saying HTTP can talk to FTP servers as long as there is an HTTP to FTP proxy.
The only thing that makes them seem compatible is there is a well formed address space in v6 that clients send v4 requests to. But it’s still v6 and a 64 proxy needs to have an actual IPv4 address to translate the source to before sending it via v4 to the actual destination.
The same could be said of the awful mess we have currently with IPv4 NAT almost everywhere on the current IPv4 network (and CG-NAT as well).