What if the cryptocurrency comes from an exchange that no longer exists and I am unable to generate a confirmation?

In this case, it is best to use email confirmations of the deposit or withdrawal orders to/from the exchange. If this is not possible either, please describe as precisely as possible the flow of the funds being verified to enable verification of the information about them in the external sources of information related to blockchain.

Alternatively, please provide an annual tax return for the period when the cryptocurrencies were purchased to document/validate the ability to purchase them.