You can always cash out "Bitcoin you [allegedly] bought back in 2010", or Monero you mined back in the day so there's no way to even prove its source, and declare that to the IRS. With a cost basis of almost zero, you'll pay long term capital gains tax, 15% or 20%.

BTW when you declare your crypto gains, the IRS not only does not care about the source, there's not even a place on the forms to list the source.

That might change in the future though.