Great question with real tax implications. This is not a zero cost basis situation — you still have your original purchase records, so your cost basis is whatever you paid for the token.

The key distinction: if the blockchain shut down and the token is truly worthless and unrecoverable, you may be able to claim a capital loss (worthless security/abandoned property). But if it arrived at the exchange, it was a transfer, not a disposal — no taxable event occurred at that point.