> It’s still an abstraction
C is still heavily abstracted. Modern OS evolved in conjunction with C s.t. it behaves like a C runtime simulating a PDP11 (I remember reading a nice article on that). To learn system, what you need is an OS course, not C. I'd say the current C-based stack is in a quite embarrassing state.
I think this PDP-11 meme is quite misleading. There is nothing really in the C programming model specific to the PDP-11 and C was used on systems much weirder than a PDP-11. C is also very successfully the basis of many parallel programs running on multi-processor systems with a memory model in the standard that was created much later than the PDP-11.
Also somehow the implied argument that computing hardware and operating systems simulate a PDP-11 for the sake of C is completely backwards. Historically, there were other approaches, e.g. processors designed for object-oriented programming or actors etc.. All those were not very successful.
It's clear that the authors were either inspired by the PDP-11 ISA and/or designed the language to make porting UNIX from PDP-11 Assm to it easier.
UNIX for PDP-7 in Assm -> UNIX for PDP-11 in Assm -> UNIX for PDP-11 in C
https://news.ycombinator.com/item?id=42644851
and
https://news.ycombinator.com/item?id=43245166
For someone just starting to learn CS, it should not matter whether they're on PDP-11 or x64 or ARM. You can't start teaching with pipelining and speculative branch execution. So C is good choice because there is a simple CPU architecture that it maps to well.
Probably thinking of this article: "C Is Not a Low-level Language: Your computer is not a fast PDP-11."
https://queue.acm.org/doi/10.1145/3212477.3212479
Sure, you need an OS course, but that's when students should learn C as well.