It's interesting how some ideas very quickly start popping up everywhere at once. One of my colleagues was working on adding support for fibers to systemd for some time (and the PR was merged a few days ago[1]!). From my understanding, this model is really quite useful for programs like pid1 that cannot use threads.

[1]: https://github.com/systemd/systemd/pull/39771

Funny that systemd appears to copy features from GNU Shepherd now after years of the opposite.