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.
Doing it via systemd is like drawing a big black box, writing LINUX on the side, and calling it a day.
I for one will not be strong armed into systemd or any other tech. If KDE makes it impossible for me to run without systemd, it goes into the trash bin. I will just install Trinity (KDE3) and be done with it. (Gnome deserves no consideration whatsoever.)
Which is why I asked "learn about what stuff". I think if the goal is to learn about "Unix" or OS design/ideas, you're better off with a leaner, "pedagogical" OS, like xv6. If the goal is to piece together an OS and really understand each piece, I don't think you really want sysvinit. You want something closer to an /etc/rc.local that just kicks off a few daemons and hopes for the best.
You can argue that sysvinit makes a better "compromise" between usability and clarity, and I'd entertain that idea, but then I think dinit is far easier to understand than sysvinit. And of course, at that point you can shave yaks till you fill the bike shed with wool.
Realistically, as much as people may hate it, if you have to pick a single init to standardize on for clarity and "building an entire Linux distro from the ground up, understanding how every piece fits together", systemd is the most rational choice. It's the most representative of the ecosystem, and requires the least "extra layers" to make the "desktop layer" work.