Learn about what stuff? Linux? System V UNIX?

I haven't done LFS since my tweens (and I'm almost 30 now), but I remember the sysvinit portion amounted to, past building and installing the init binary, downloading and extracting a bunch of shell scripts into the target directory and following some instructions for creating the right symlinks.

Obviously, you can go and check out the init scripts (or any other individual part of LFS) as closely as you wish, and it is easier to "see" than systemd. But I strongly protest that sysvinit is either "Linux" (in that it constitutes a critical part of "understanding Linux" nor that it's really that understandable.

But setting aside all of that, and even setting aside the practical reasons given (maintenance burden), when the majority of "Linux" in the wild is based on systemd, if one wanted to do "Linux From Scratch" and get an idea of how an OS like Debian or Fedora works, you would want to build and install systemd from source.

For me, Linux From Scratch is not about compiling linux from scratch, but on building up an entire Linux distro from the ground up, understanding how every piece fits together.

Doing it via systemd is like drawing a big black box, writing LINUX on the side, and calling it a day.