Have previous protocols (TCP, UDP, etc) been in widespread use before their respective RFCs? 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-tra…
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.