There is openrsync, which the OpenBSD re-implementation of rsync

It's not a fork, but it's 8 years old, and is already shipped by default in OpenBSD and macOS.

To quote Tridge:

> As to all the people saying “I’m going to package openrsync for platform XXX and we’ll use that!”. I find that rather amusing. If you do decide to go down that path I’d suggest you try the new rsync test suite on openrsync if you can stomach something that an AI has helped write. I tried it today and openrsync currently fails 85 of 98 tests, so I’m sure it won’t take you long to get it up to speed. You run it like this “./runtests.py — rsync-bin=../openrsync/openrsync — use-tcp”. Admittedly a lot of the failures are just features openrsync doesn’t have, but still, it’s not a great result.

It's also not feature-equivalent to rsync. But if it meets your needs, it's an option.