Internationalised domain names will display fine because they are punycode that browsers and other clients happen to render as internationalised domains.
In my experience Punycode support in browsers varies greatly depending on the phishing[1] mitigations in place, making them quite unreliable in practice unfortunately.
I'm pleasantly surprised about SMTPUTF8 being a thing though. Yeah nobody uses it, but I thought email had zero UTF-8 support so it's at least better than nothing, I guess.
E-mail is even worse than web applications at this because even common and popular mail clients can't do UTF-8 in email addresses right (https://www.google.com/search?q="SMTPUTF8"+site%3Adovecot.or...).