I was just reading the QUIC multipath RFC. Didn't it come out literally yesterday? I guess it's common to have the implementation foreshadowing the RFC but it's jarring to see them back to back like this.
Many QUIC features get implemented while in draft stage as people iterate on the RFC's design (e.g., ACK Frequency[0] is currently on draft version 14 and I implemented support for it three years ago in quinn).
It’s pretty common for IETF drafts to be substantially complete well before they are finalized as RFCs. For example, supporting ML-KEM in TLS is still a draft, but there are already multiple large scale deployments of it since the technical aspects were nailed down a while ago
It’s common enough for groups to not consider the spec done until there is a decent bit of implementation experience (both because actual implementers tend to find interesting bugs in specs, and because many things are hard to measure without any implementation).
[0]: https://datatracker.ietf.org/doc/html/draft-ietf-quic-ack-fr...