I got that notification a few days ago: "Reserve your username!" Every username I tried - based first of all around my very-uncommon real-name" - that did NOT have numbers at the end of it was either already taken (fair enough) or else "reserved for Business account holders", and Would I like to upgrade? It's a monetization push.
I think this is a very useful feature if the users still have the option to exchange phone numbers if they want.
I trust whatsapp like a fart in flu season. Use signal and encourage your friends to switch.