https://labs.spotify.com/2013/06/18/creative-usernames/
I had someone tell me that programming isn't real work before, and this is yet another example of all the small little details going into building things that most people don't really think about day to day.
I haven't had to work with login code in a while, but might at some point. I know some systems only allow alphanumeric usernames, but sounds like can't force people to have alphanumeric emails... Well I guess you could but might upset someone. I know there's normalizing functions though like NFD, NFC, NFKD or NFKC that might work for usernames but not sure what's really recommended.
Then also brute forcing attempts to try to mitigate attacks and other considerations to make also when building out an account system. Then if your company is large enough to provide phone support, not sure how you'd tell the support person which specific emoji you used in your username.