The PostgreSQL frontend-backend protocol supports protocol negotiation, so that if the client supports a higher version of the protocol than the server, they can agree to use the lowest common denominator. In practice, that never happens because there haven't been any new protocol versions since that mechanism was introduced. The protocol version is always 3.0. The same mechanism can also be used to negotiate the use of optional protocol extensions. That is also not used in practice because no optional protocol extensions have been introduced.

There were some patches in the v17 release cycle to introduce protocol changes that would've used that mechanism, but they were not committed. For future-proofing, it would be nice to support the protocol negotiation in the proxy.

See NegotiateProtocolVersion at https://www.postgresql.org/docs/devel/protocol-flow.html.