Meanwhile, the CMake files for #systemsettings make no mention of the #KUserFeedback library, I know, I looked and even built it from source myself, to check.
Turns out that #KUserFeedback gets optionally pulled in by #plasma-workspace, so if you build that and then build #systemsettings against it, suddenly, the User Feedback entry appears in systemsettings5.
As if by magic (;*
But, it's not a build dependency of #Plasma-workpace , apparently, so the #BackportsPPA for #KDE doesn't need to include the #KUserFeedback library, but will pick it up if it already exists.
Recently the PPA updated to 5.18.1 and now systemsettings5 has the entry for User Feedback without my needing to rebuild it and plasma-workspace from source.
So, that's a bonus (:*