Is there a command line interface to Keychain's / the system Password Assistant utility?
The "Memorable" password generation option appears to be gone in MacOS Catalina, there is now only;


Manual
Letters &amp; Numbers
Numbers Only
Random
FIPS-181 compliant


The security command line tool doesn't seem to generate passwords, it will only accept one you define. If there's another command line tool for password generation built into the OS that re-uses the above options, I don't know about it.

Given the combination of both of those factors, I'm documenting and accepting this answer as closure.