Most modern Western currencies were previously backed by gold (and/or silver previously). The German mark and other European currencies were replaced by the euro, which is still around.

Many of the above only left the gold standard in the 1930s, so they're still around but have only been fiat currencies for around 90 years. Some European currencies have been off the gold standard for several centuries.

I'm not aware of any major modern currency that ever expired (the change to the euro wasn't an expiration because holders of the old currencies got euros instead).

Does that mean that no fiat currency ever expired? Of course not. If a country with fiat currency was conquered, its currency likely went belly up with it. Even currencies backed by gold though may have expired when the conquerors looted the gold.

On the flip side, Bitcoin and other crypto currencies are also all fiat currencies, as there is no gold or silver backing them. Those that claimed to have dollars backing them (note the irony there) have been found to be based on a lie and a fraud.

The thing is, crypto currencies don't have even the ability to tax (a la the US dollar) backing them. If Bitcoin goes the way of Terra, FTX, and many others (https://fintechnews.sg/67859/crypto/top-10-biggest-crypto-failures-of-2022/), and can't afford to cover an exchange to dollars or other real currency, you have zero recourse.