I think systemd has changed everything. POSIX, and the UNIX philosophy are dead now.
It is still worth to learn i think.
Here is a classic video I found entertaining showing AT&T Bell Labs demonstrating UNIX of old. The inspiration behind the Linux of today. It features interviews with Dennis Ritchie & other programmers/original UNIX creators.
I find some of these old videos fascinating. 37 years ago (1982 video, UNIX created in 60’s) and commands are still the same.
It is more like How to use Linux than go deep into understand *Nix and learn to control / develop it but it is really really useful. Specially grep, pipe and find magic commands