All of the links I've conjured up are pointing to the same bit of information, will continue to hunt down more but I can say at least for my account I was able to fully delete my FB account, and login to the browser version of messenger.com using the same email/pass combo.

I've also been able to log into the Messenger mobile app without giving them a phone number (I was prompted, but not required at the time to enter a phone number). This all happened back in July of last year, so if the mechanism has changed since then, it's entirely possible you're correct that a phone number is now required instead of optional, as it was for me-and everyone else before the change and we just got 'grandfathered' in. Maybe?

It’s a deliberate pig to delete but the pretence of deletion is there.