I have always wondered how the tax department will react in case you suddenly start reporting your gains. Is it even smart to report your gains after not having done it for years? I have been thinking about it for a while.
Can you just claim that you found back an old wallet file? At some point large holders will become multi millionaires and it will somehow require them to come up with a solid plan of action to buy properties and such.
That's actually a good question, but it depends on what country you are from firstly. I have done some research on this last year, and the explanation was pretty simple and straightforward.
If I can prove that my coins haven't been into circulation in the previous years, which can be proven easily with how open the blockchain is, it's like I instantly obtained that wealth, and only have to start paying due taxes from the moment I claim that I gained back access. That's from a wallet backup, which I assume is the easiest part because it clearly shows that your coins haven't been moving. I don't know how things are in case you can't prove that your funds haven't been into circulation.
In case of the latter you are better off just not declaring anything, or you have to contact an attorney specialized in crypto tax cases to see what your options are.