Not something I've done even in my many years with Drupal so I'm not sure what the best approach might be. Looking at things, D7 may have offered some modules (like the message + message notification module) that may have helped, but they don't seem to be ported to Backdrop so I'm trying to figure out another way.
Basically, I'd like to (as the admin) be able to put notes of some sort on a specific users account that they (and only they) will see when they log in. I need them to acknowledge these notes somehow (link/checkbox/whatever) and force some actions and/or prohibit activities until they have.
For example, I might put a note saying on user 32's account that "There was a problem with your account, please call the office." That would show on their user page until it's been acknowledged, and ideally I could hide a menu if any notes have not been acknowledged (if the latter part is too much to ask for it should at least be a constant nag at the top of every page.)
If anyone has any suggestions it's much appreciated!