There is a reimplementation of xz here at least https://github.com/gendx/lzma-rs

None of these suggestions actually cover the part where someone or someones do a bunch of bloody work.

Again, this is the problem in the first place.

At least in Debian there's already the inevitable discussion about a supposed opportunity for "finally" moving to zstd. Which to me, frankly, feels a bit like getting out of the frying pan and into the fire, or what I think Arch (yuck) did years ago. That's not because I'm so much into xz. It's kind of funny there's a possible Debian connection, of all things, considering that quite a lot authorities and services in areas like China, Russia or Iran are recently migrating, for obvious reasons, from Windows/Mac to more or less home-made (~styled) Linux distributions, that just often happen to be based on Debian. I don't think this is too popular in English speaking countries in particular, but then I really didn't want to chime in on the speculations. ;P