That's my question. A person who lost his private keys for years and recovers them. Maybe he mined btc or he won btc with a faucet, how can he prove all that after 10 years ?
Ownership can be proved in a very simple way, and that is by signing a message with one or more public BTC addresses. After that, he has to find out the process of paying capital gains tax, and then he can sell his BTC without any problems.
If I found 10 BTC and wanted to sell them all I would have to do is submit a tax return and explain when and in what way I acquired that BTC, and for all crypto assets acquired before 2016 I do not pay any tax (local law). I don't think this kind of thing is a problem, at least not in countries where BTC is not declared illegal or if it is not acquired illegally.