It is a rich tapestry of feelings to read through the proc(1) manual page from Solaris 2.5, dated literally 30 years ago in 1994:
https://sysmgr.org/~jclulow/sol25/proc.1.html
And then compare it with the modern equivalent in illumos:
https://illumos.org/man/1/proc and https://illumos.org/man/1/ptree