> Still doesn't change that rexxsyslib.library
> does not exist on Aminet.

Still doesn't change that here clicking the link I provided makes Aminet say "Found 22 matching package(s)". Piru says it works with him as well:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7158&forum=3&start=2

> Doesn't that still have to be obtained from the
> OS3.x distribution?

If you want the latest version, yes. Else what Aminet has since 1995 should be fine.