The old Linux system api has not been changed much. It’s still pretty much the same. The new addition doesn’t obsolete the old stuff. That’s why the book is still relevant.
https://abi-laboratory.pro/?view=timeline&l=glibc
Correction: https://manpages.debian.org/testing/manpages-dev/syscalls.2.... is listing some system calls as removed. I didn't know that. Strange things happen at sea.
Also: things did disappear from procfs.