I specifically got a custom domain and email address for any non-personal/"professional" comms, which is essentially just me@<custom-domain-featuring-my-name>.com.
At least with non-ASCII characters in passwords, while I think it is stupid to not handle those properly, I can at least see some sort of an excuse there, no matter how weak it is. All it takes to mess this up is not thinking about handling those scenarios, so I can definitely see "this issue was created due to us not thinking about this possibility or not willing to deal with handling it."
But what's even the reason to not allow sub-3-character local portions of emails? How does one even mess those up, aside from intentionally setting some triggers for less than 3 characters in local portions of email addresses?