If I was the authorities, I would insist that you prove your loss of bitcoin, I will not be convinced otherwise.
But that's just me.
Discussion
how do you prove that
I think it's almost impossible to prove
A tax authority will just assume you have made 100% profit unless you can prove otherwise
i belive the question is how to prove you lost it
you can always prove you bought it and so the cost basis. so how they assume you made 100% profit
If the question is still how to prove you lost it then it really doesn't matter how much profit you made on it. What matters is the loss of bitcoin.
In addition, you don't need to prove you bought it became it is assumed, for argument's sake, that the BTC was bought with KYC.