I have been dealing with a similar problem: Very old holdings that I first traded for back in 2013. It's unfeasible to record what I traded then since it goes through a trail of dead exchanges. I've been filing them as capital gains as I trade them now, dating their purchase time back to the year I originally traded for them(thus making them long-term), and their acquisition price as $0.

Marking acquisition price on trades to $0 is consistent with how I've seen the IRS behave in the past when I failed to report stock trades accurately($1mm volume = $1mm taxable when this occurs), and since the prices have increased 1000% in most instances(I buy and hold for years, and while I traded in 2017-2019, I did not trade at all in 2020) the initial cost is easy for me to write off.

I plan to use Cointracker's reports going forward.