it's an interesting use case, but i think it is more negative than positive. for example, it makes it easier for scammers to impersonate someone if you can have multiple nostr addresses. all someone would have to do is spin up a few domains and use a few identity providers and they could have many aliases. some may think the more the better or the more they have, they more legit and that's simply not the case.

to me it sounds like you want to display nostr badges as gamer tags. they were designed exactly for this reason. nostr:npub1qqqqqqyz0la2jjl752yv8h7wgs3v098mh9nztd4nr6gynaef6uqqt0n47m created them as a method to cosmetically display somemthing on a user's profile. i feel like you should go this route.

anyways, depending on the client, you've always been able to search for non-active nostr addresses. they wouldn't show as active or / valid though.

for example, im still derekross@nostrplebs.com and before that i was @derekross.me. you can search for those addresses on some clients.