The Soviet Union and East Germany ceased to exist What's happened to their debts?

When states cease to exist, under international law, they normally have a successor state that takes the place of the earlier state. The successor state gets all property and privileges, but also all debts and liabilities of the state it replaces. Of course, that debt is usually renegotiated or rescheduled, but it doesn’t just vanish.

In the case of the Soviet Union, the successor state is the Russian Federation, though some of the property and debt was distributed amongst the other 14 successor states (Ukraine, Kazakhstan, etc.). Hence the Soviet nuclear arsenal went to Russia along with the Soviet permanent seat on the UN Security Council. That said, the other former Soviet republics (Ukraine in particular) have been unhappy with the arrangements and the succession is not entirely settled.

In the case of the German Democratic Republic (East Germany), the Federal Republic of Germany took over all East German state property and liabilities. That included all military hardware — so for a brief time, there were Soviet-made MiGs and Sukhoi fighters with West German Bundeswehr markings. Some of them are now parked at a museum about a 30 minute walk from my house, though the markings have mostly been removed.

A former East German MiG-29 with West German Bundeswehr markings in a Berlin museum