''As a personal note, I do not like this decision. To me LFS is about learning how a system works. Understanding the boot process is a big part of that. systemd is about 1678 "C" files plus many data files. System V is "22" C files plus about 50 short bash scripts and data files. Yes, systemd provides a lot of capabilities, but we will be losing some things I consider important.
However, the decision needs to be made.''
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.