Yep: YouTube currently delivers over gQUIC and h3 (IETF QUIC).

We advertise the following alt-svc for most devices:

> h3-29=":443"; ma=2592000,h3-T051=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"

HTTP/3 (over IETF QUIC) is still not as widely deployed due to lesser client support, but that's growing as both Chrome[1], Firefox and Apple (in iOS Safari, when enabled) adopt it and the latest drafts.

[1]: https://blog.chromium.org/2020/10/chrome-is-deploying-http3-...

I think that indicates more about the economics of YouTube than anything. Probably their frontend CPU costs would have to go up by a factor of ten thousand before it even began to approach their costs for compression and storage of user uploads.

> I'm pretty sure YouTube uses QUIC for video streaming today

Is there any media transfer protocol based on QUIC?