A better way to think about it is that Tor routes streams. Tor has the concept of "pluggable transports" which don't care what form the data takes over the network, as long as a stream can be reconstructed.

For example, Snowflake[1] which is WebRTC based, and meek[2] which chunks the stream into a series of HTTP requests/responses for domain fronting. TCP is still used between nodes.

[1] https://trac.torproject.org/projects/tor/wiki/doc/Snowflake

[2] https://trac.torproject.org/projects/tor/wiki/doc/meek

AFAIK the proposal for HTTP-over-QUIC was to race with a TCP connection. Similar to Happy Eyeballs for IPv4/IPv6 dual stacks (meaning we could end up with 4 connections attempts?).

Exit nodes would be the only part that needs to support it.