"What should be done about airdrops from tax years prior to 2019 if you didn't calculate basis and income this way?"
I don't see anything in the IRS 2019-24 that addresses this. We take our cues from them. They are always a little slow to play catch-up. An event happens and they have to write the tax law on how it is addressed. It happened back in 2000 with day traders. At the time there was nothing that addressed it and then the IRS started coming out with a plan. Unfortunately, we have to wait for them to address every aspect of it.