[WorldMaker]

HTTP/3 replaces parts of OSI layer 4 (TCP/UDP). There's a perspective that HTTP has been trying to lower its layer for decades. Especially the way that TLS originated as HTTPS but now is almost a "universal" vertical chunk of Layers 4, 5, and 6. In that perspective HTTP is probably the new Layer 6.

[jeroenhd]

HTTP/3 uses QUIC as a layer 4 equivalent (which is actually just UDP with extra implications in practice). Not many services leverage standards-compliant QUIC other than HTTP/3, but you don't need to do HTTP to get the same protocol working.

If anything, HTTP/3 running on top of QUIC forced shitty middlebox vendors to de-ossify by permitting any QUIC-based protocol, as they cannot practically distinguish a new HTTP/3 connection from a QUIC connection.

[afiori]

My understanding is that they cannot even really recognise a QUIC connection (maybe the first time, but not after ip roaming)

[thequux]

Ehh, not really. OSI layer 5 was responsible for managing multiple non-overlapping sessions within a single transport stream, and routing those sessions to specific applications. This is precisely what HTTP/1.1 did (though the accept, content-type, accept-encoding, and transfer-encoding headers are really an implementation of layer 6); QUIC, on the other hand, covers the same layers that TCP does (3-5) plus the aforementioned layer 6.

I recommend actually reading X.200 (the specification of the OSI model) at some point: it's quite approachable (especially for an ITU spec, which are notoriously dense reading), and will quickly make you realize how silly it is that we still use it as a reference for modern stacks.