Please consider that today, in 2021, IPv4 is still the only thing supported in many venues. It took almost 20 years for IPv6 to become universally supported by Operating Systems, ISPs, providers, etc, but still you have to have IPv4.
And I dare say IPv6 was much much more critical to have than QUIC.
But having said all that, what is there to allow in QUIC? Isn't it just using UDP?
Your aren't wrong about that, given the amount of fallback browsers do, but IPv6 actually required upgrading key pieces of hardware at critical points in the infrastructure[0]. Not sure if there's any hardware released in the last 10+ years that doesn't support it, but who knows?
... and yes, QUIC is "just UDP".
[0] ... and dare I say: a more compelling immediate use case. At the time is was designed the address shortage wasn't actually looking all that dire. It might actually have been invented/designed too soon... like it solutions to Y2K had come out in 1980 or something.
If browsers fall back to regular HTTPS when the QUIC connection fails there won't be much incentive in making sure QUIC works or isn't blocked.
Yes, but there‘s still networks blocking everything but TCP 80/443.
Is udp/53 blocked too?