[pedrow]

I think https://wiki.c2.com/?PlsLanguage is informative. PL/S was more suitable for writing low-level code due to some extra options and it didn't need a full runtime environment. Wikipedia suggests IBM didn't (initially) release it because it meant that only they could alter the operating system, and also to avoid handing any advantage to their competitors.

[pmcjones]

Correct. We used PL/S for the RSS, which was the lower layer of System R, the first SQL database. See https://mcjones.org/System_R/ and https://mcjones.org/System_R/SQL_Reunion_95/index.html