The burden of proof is on you (which is why I said "government treats you as a criminal by default"), and I doubt "it's from a dead exchange's gains" will cut it, even if it's the truth, which is why im worried about cashing out these coins, or coins which I mixed back in the day to text a mixer. Now there aren't clear traces about these coins.
So this thread has been ongoing for quite some time now. What have you learnt and what are you going to do? You seem to be locked into a particular spiral of doom and nothing anyone says is talking you out of it.
I have learned that I still haven't seen anyone cash out a relevant amount of signature earnings (let's say above $10000) at least in a place that's relevant to me, so I will remain skeptic and just hold it and see how I go about it in the future.
Im still worried about the mixed coins that I did mix to test a mixer out years ago (years ago 1 BTC was worth like $200) and so Im worried about cashing these out then not being able to explain where they came from and getting it counterfeited. Same goes for BTW that I withdrew from dead exchanges or lacking information of the trades in any way. Im not sure if I will ever be able to cash out any of these... so I may have to stick with signature earnings and other coins in which I can clearly trace the origin, otherwise it's extremely risky to cash them out because you don't know what the hell to expect.