Blog post: When su replaced login for becoming another Unix login https://utcc.utoronto.ca/~cks/space/blog/unix/SuAsLoginReplacement
tl;dr: System III, and then this was half-adopted by 4.3 BSD. But in the process I learned that up through V6, su only let you become the superuser. Only in V7 did su let you become any UID.
Sparked by @simontatham 's https://www.chiark.greenend.org.uk/~sgtatham/quasiblog/tron-legacy/
That got onto Hacker News and in turn got me reminiscing about the rather different way that #login was planned to work on GNU Hurd, with one running login *after* starting a shell session, adding credentials to running shell processes. It was in early blurbs for Hurd.
I thought that they never implemented it. It turns out that for a brief time at the turn of the century the #Debian #Hurd people, at least, actually did.
https://groups.google.com/g/linux.debian.ports.hurd/c/2rCbPlXXzfQ/m/4Wmg_jf8FC0J
Chris Siebenmann
JdeBP