I just published an I-D that defines a mechanism for QUIC endpoints to discover their (public) IP address and helps them determine their position in the network (e.g. if they're behind a NAT): https://datatracker.ietf.org/doc/draft-seemann-quic-address-discovery/ This is especially helpful for QUIC nodes running in a p2p setting.
A similar result could be achieved by using STUN on the same UDP socket, but there are several advantages of doing it inside of QUIC. See the draft for details.
