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?
Actually, dual citizenship in Germany is only allowed for a few select non-EU countries. For everyone else: first you apply for citizenship. Then they say citizenship will be granted, under the condition that you provide proof of revoking your previous citizenship within 2 years. Then you revoke your previous citizenship and give the confirmation to the immigration department. They will process it (during those weeks you are practically stateless) and grant you citizenship on this basis. Source: did this three years ago
As of the end of June they significantly relaxed the rules around the path to citizenship, including dual citizenship. Anyone can now do it if the other country also allows dual citizenship.