I think by the time a crypto reaches worthless value any reputable exchange will delist it.

I've held cryptocurrency that reached zero. For example, HamRadioCoin. Development stopped maintaining it, the exchanges dropped it, the official site's domain registration lapsed, Google Play removed the mobile wallet and the HAM blockchain stopped completely.

I can't think of any specific examples but since cryptocurrencies are open source (with a few exceptions) it's possible that someone with the right skills and tools can resurrect a crypto from the dead but it also would need public support. Once a crypto reaches zero value some of the largest investors will feel burned and that would probably taint a crypto and keep it a low value.

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