not sure if originally released as such, but at one point, part of a larger collection:

https://docs.freebsd.org/44doc/

https://www.netbsd.org/docs/bsd/lite2/usd.html

(missing papers were redacted due to USL vs UCB suit; most are floating around the net if you google hard enough for them)

was also released in in spiral-bound print form through USENIX for at least 4.3BSD (and I think one or both of 4.2BSD or 4.4BSD).

if you see references to 'USD' or 'PSD' or 'SMM' in manual pages, they are referring to this.

shameless opportunity for a personal take:

Unix from v7-4.4BSD was always a self-contained and self-hosting source+binary+documentation collection with a common source lineage to bell labs and its surrounding culture; therefore opengroup certified posix is not unix, despite whatever copyright law might actually say.