Comment by billpg

Shameless plug: https://billpg.com/sixgate/

(Short version: IPv6-only networks can provide a gateway for IPv4-only clients and advertise it via reverse DNS.)

SixGate has an interesting contrast with this RFC. RFC8950 removes IPv4 from the network core while preserving IPv4 reachability, whereas SixGate removes IPv4 from the service side while preserving reachability for IPv4-only clients.

In both cases, IPv4 survives as a compatibility layer rather than a fundamental requirement. The difference is that RFC8950 is aimed at network operators, while SixGate is aimed at the last remaining IPv4-only users.

In both cases, the goal is the same. Turn IPv4 from a requirement into an optional compatibility feature.