I needed to use my previously-deactivated Facebook account to help my mother set up Facebook Messenger for dumb reasons, and I was able to log into Messenger, so I guess my account was not quite fully deactivated but still in that Deactivated Except Messenger (DEMA) state.
Except that things in DEMA are a bit…wonky. Trying to search for new contacts didn’t give me the list of filtering fields to narrow them down. Opening the “control your visibility to others” option took me to a Facebook login that doesn’t actually reactivate my account, but gave me an extremely-stripped-down Facebook interface that didn’t actually let me adjust any settings. All of the instructions I could find about adding contacts refer to fields and menu options that just weren’t there.
In the end I wound up having to fully reactivate my Facebook account just so I could actually add a new contact to Messenger. And, having been exposed to the full Facebook UI and feed, I am now depressed again. Hopefully it doesn’t take too long to get my mother’s stuff sorted out so I can deactivate it again ASAP.