Well, you can’t use them because of attitude like yours. You’re not only annoying those of us who want to use plus signs or international domains, but ensuring the “exotic” half of the world that doesn’t use the Latin alphabet is kinda left out.
Non-ascii alphabets need a lot more support than just accepting them. But I wouldn't really consider plus signs exotic anyway.
What does that even mean? We shouldn’t support them because it requires more work?
If you are consistent. A couple times I've successfully signed up for something with a username+string@gmail.com address but then have been unable to unsubscribe because my address is "invalid".
I kinda recall a lawsuit many years ago about a unsubscribe confirmation email.
in fact it probably catches more mistakes than deliberate weird email addresses to be stricter than the standard mandates