Now more recently I started receiving my first "hey we believe there is something wrong with your account, attach a (first time, new) phone number to it to join this server". That is most likely a lie of course, Discord just wants to blackmail me into giving them my phone number. Also apparently there is an option to make an attached phone number mandatory in order to join a server, which quite a number communities activate for some reason.
So it turns out the bad feeling was correct.