The QUIC Wikipedia page[0] makes it sound like a substantial amount of traffic, due largely to Facebook, Google, and the Google Chrome browser, uses QUIC already. Facebook claims 75% of their traffic (for the app?) is via QUIC[1]
[0]: https://en.wikipedia.org/wiki/QUIC
[1]: https://engineering.fb.com/2020/10/21/networking-traffic/how...
As other people have noted, yes.
One of the interesting facts about the IETF is that they actually require two existing implementations in order to advance from an RFC to an official "Internet Standard":
The IETF Standards Process (RFC 2026, updated by RFC 6410) requires at least two independent and inter-operable implementations for advancing a protocol specification to Internet Standard.
I suspect that this requirement for two independent implementations that can actually talk to each other is one reason the IETF has a relatively solid track record.
In general, yes: modern RFCs should address existing practice.
The very early RFCs were just that: requests for comments, i.e. memos. Sometimes reflection on existing practice, sometimes design discussions.
For your specific examples (TCP and UDP): they were part of a redesign of the Internet protocols (well a redesign of the ARPANET's protocols TBH, NCP, with the intent of enabling smooth internetworking). So those RFCs reflected the specification of new protocols, though some experimentation had been done.
I remember the transition day. The first machines, IIRC were ITS machines that some had hoped would not make the transition at all! I don't believe there were any Unix machines on the net that day -- Berkeley sockets hadn't yet been released.
Yes. Many early RFCs were simply documenting things as they were, not specifications of how they should be.