The network layer can already be used for tracking in multiple ways, including HTTPS sessions, ETags, and cached files with identifiers. When browsers partition the network layer they need to partition connection state as well, which includes QUIC/HTTP3 state. Safari already does this, and it looks to me like Chrome and Firefox are doing it too: https://www.jefftk.com/p/shared-cache-is-going-away
(Disclosure: I work on ads at Google, speaking only for myself)