I'm pretty sure it's accepted pretty universally that countries must accept citizens back. Reason being, if they don't, the rejected person becomes another country's problem, and that is bad for relations.
It's unconstitutional to deny a US citizen entry to the US. It's the same in most other countries. But it still happens in many countries; Sweden for example has had a lot of problems deporting convicted criminals to their homelands, because their homeland refuses to accept them back.
What citizenship? I never saw his long-form birth certificate. Clearly, he's hiding the truth that he was born on the moon. Musk should pilot Starship to take him back on a one-way trip.
Remember when Trump said he sent investigators to Hawaii and told us "you wouldn't believe" what they were finding? And then, I guess, decided that was literally true since he never told us what he decided they had found?