The following is also a nice resource with some background information that describes the FSSTND ("Filesystem Standard", precursor to the Filesystem Hierarchy Standard or FHS)
https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/index.html
/usr usually contains by far the largest share of data on a system. Hence, this is one of the most important directories in the system as it contains all the user binaries, their documentation, libraries, header files, etc.... X and its supporting libraries can be found here. User programs like telnet, ftp, etc.... are also placed here. In the original Unix implementations, /usr was where the home directories of the users were placed (that is to say, /usr/someone was then the directory now known as /home/someone). In current Unices, /usr is where user-land programs and data (as opposed to 'system land' programs and data) are. The name hasn't changed, but it's meaning has narrowed and lengthened from "everything user related" to "user usable programs and data". As such, some people may now refer to this directory as meaning 'User System Resources' and not 'user' as was originally intended.
https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/usr.html
But it gives no additional source for that claim.
One more thing I could find is this: https://marc.info/?l=openbsd-misc&m=127276011001745&w=2
But that mostly just says what we already know (that user directories were located in /usr
). Once again only circumstantial evidence for the origin of the abbreviation.
Then there is that one Rob Landley mail, which is also mention in the "Unix directory hierarchy history" article Harvey linked, which confidently claims:
I suppose Thompson and Ritchie just saw it as self-evident because I can't find any direct quote or note from them describing the name choice. I could've sworn I saw something like that at one point, but I certainly can't find anything now.
All in all I'm pretty sure /usr
meant "user" in the beginning, and because that is confusing with how those directories are used nowadays someone coined the "Unix System Resources" backronym to draw attention away from the original meaning and have people focus on the (new) de facto meaning of the directory.