I believe it's also a regulatory problem. Plankalkül on a German site makes perfect sense, because that is a German word. But then there would be millions of ways to form domains that serve no other purpose than misleading the users. So registrars would need to make sure that all letters belong to the same script and make sense i the language(s) native to the domain.

But then this is the internet and greedy and incompetent registrars are a fact, so I am not sure this will ever happen.

As for the UX, maybe displaying a little flag or similar emoji indicating what script it is. And showing a big warning or completely blocking the site of the user has not accepted the script in question. That is for the whole domain, mixing scripts in a single domain should be massively limited and requires other indicators in foreground or background. Also a problem for the color blind.