A citizen is a citizen. You don't create second class citizen, who can see their rights revoked at any time, whereas the other class never has to fear anything like that. That's basic democracy 101.

Multiple citizenship is handled with quite a bit variance over the globe, including those we call democracies. - https://en.wikipedia.org/wiki/Multiple_citizenship

Citizenship obligations for multiple citizenship is quite interesting from a legal perspective. Some citizenship require taxation regardless of where one live, which makes it possible to have a requirement to pay taxation to multiple countries. Military service is also an other interesting aspect. Giving people with multiple citizenship more rights than people with single citizenship would naturally cause problems, and yet you don't want citizens with different rights as that would indeed create different classes of citizenship.

A person can also end up with one citizenship in a country that allows multiple citizenship, and one which does not, and the citizenship of the person then depend on which country is being asked.

The inventors of democracy, ancient Athens, specifically had a vote to ostracise (kick out) citizens.

They also had slavery. And didn't let women vote. And lived over 2000 years ago. Why hold them up as a model to follow?

I didn't. I was simply pointing out that not kicking people out has nothing to do with "basic democracy".

If anything it's basic democracy that if 51% of the population demands someone is kicked out, they're kicked out, no matter how unjust.

Democracy means doing what is popular, not what is right.