I searched the forum for backward compatibility and didn't find anything.

So, iPrint client:
v 5.82.00

(32-bit Windows 7) - not that it matters, IMO.

Upgraded OES11 SP2 server to OES 2015 SP1 and applied post patches.
Now, nobody can install an iPrint printer from the server unless they upgrade to the iPrint client that's included with OES 2015 SP1.

Don't recall running into this issue before with prior OES versions, but wasn't sure if some change to backwards compatibility was made, or if it's some odd config setting that needs to be done on the server.