/proc is not Linux-specific; my late, great Colleague Roger Faulkner did a lot of work on it in Solaris:
https://www.usenix.org/memoriam-roger-faulkner
...and there's a general history here:
https://blogs.oracle.com/eschrock/entry/the_power_of_proc
The part that may be specific to Linux is Linux provides a text-based interface, whereas systems like Solaris provide a binary interface.