Do the old websites get their domain rentals revoked?

TLDs can exists long after a country, the Soviet Union's .su still exists.

If two countries merge legally then deciding what do what to do with the TLD would be part of the agreement or at least dealt by IANA and the merged country.

If it's an illegal occupation under international law then the TLD would be probably be managed by some kind of government in exile.

Basically yes, but it won't happen immediately or quickly. It's very unlikely any domain would be revoked just like that, but they might eventually stop registering new ones and accepting renewals. ICANN has a retirement policy that they will follow.

Interestingly, this is relevant to the popular (among tech bros) .io domain, which belongs to the British Indian Ocean Territory, a small cluster of islands in the Indian Ocean that is currently in the process of being transferred to Mauritius. That might mean .io domains will eventually be phased out.