It's not just "accounting convenience" it's literally the way the profit vs loss of the central bank is defined. The central bank typically has a legal obligation to maintain stable prices, so while outstanding currency is not a liability in the conventional sense with a certain maturity date, interest etc., it's undoubtedly a debt towards society at large, which could presumably need to be redeemed and sterilized in the course of monetary policy, at least in part. Outstanding currency is a perpetual zero interest loan towards the central bank by the holders of currency.

If a central bank loses 80% of its real reserves, and financial circumstances arise where it must repurchase more than 20% of it's issued currency, for example to defend the exchange rate against a capital flight, than the bank is effectively "bankrupt" - it can no longer fulfill its legal role and regulate the value of the national currency. It's not a traditional bankruptcy, but it's a de-facto failure which many central banks experienced.