That's crazy they wont take "I bought bitcoin back when it was around 20k and held onto it for years" approach! What are the probably many other people who still have bitcoin from 2013 and so on when it was only a few thousand but they bought from one of the plentiful non KYC exchanges?
So is it if you just show up with 1 mil out of no where? Or is it so long as they can see that the bitcoin has sat in a wallet for a long time but is just not connected to a paper trail, would that matter?
Either way, something tells me the totalitarian states of America is not the place to be when you want to cash out.
If you got your Bitcoins in 2013 and they are in the same direction today, you are in a good position. The only thing is that if you can't prove the purchase price, you will most likely be charged a 0 purchase cost, which won't make much difference with the real purchase price, since almost everything is profit.
If you moved the Bitcoins by mixers for "privacy" I think you have it more complicated as the IRS does not like privacy.