"Because of this, xbps-src is in fact not a binary, it is a collection of very intricate shell scripts that use standard tooling available on almost any Linux distribution to bootstrap a compiler, build chroot, and XBPS itself."

When he stepped away from NetBSD, I remember reading that it was because he wanted to work on a package manager.

It might be that the entire raison d'etre for Void Linux is a package manager consisting of shell scripts.

Not sure what he thinks of pkgsrc but IMO their work has made things so easy that, e.g., one can easily use a set of shell scripts instead of pkg-add, etc. I do this to save space on small systems. IME, the pkgsrc bootstrap scripts are a very reliable way to build a gcc toolchain. (build.sh as well)

One project I have had on the list for many years is to build Linux on BSD. Could it be done? Has it been tried?

Also, maybe S6 deserves a mention as another daemontools clone, with the added bonus that it may lead one to discover execline.

Nothing against runit. I use socklog and occasionally udpsvd by the same author.