Let’s say I become a citizen of a country that doesn’t allow dual citizenship. During naturalization, new country B tells me I have to renounce citizenship from old country A.
Does that have any effects back in country A? How would country A know? Would country A even care if they found out?
Countries typically don’t allow that. (Do any allow it?) For example, Canada requires you (at least) to be a citizen of another country and to live outside Canada.
The UN would likely consider it a violation of their human rights if a country knowingly allowed a citizen to become stateless. I would hope that at least all member states would not allow it, but I don’t know for certain.
The USA allows it. Though I imagine the embassy staff will probably try to warn you.