Even if quic would bring along an unreliable stream option: The browsers Api still only expose http semantics via xhr and fetch. So those would need to get extended too.
Actually doesn't webRTC already have unreliable channels?
WebRTC has unreliable data channels, but they are based on sctp/dtls/udp. Making them work over quic streams might be possible, but also requires a new standardization effort.
If someone does it I would actually prefer an API outside of webrtc, since that one carries a lot of complexity for signaling and requires again a few extra protocols (ice, stun, turn, etc). An unreliable/unordered client to server protocol and api could potentially be much simpler.
The current focus right now is to deliver WebRTC 1.0, when this is done, we may reconsider it.
(I'm an editor of the document and am in favor of it).