Before 2019, taxation on Airdrop tokens was fairly unclear. There were two sides that most tax professionals were on. One was that airdropped tokens were only taxable when sold, using a cost basis of zero. This followed the logic of stock splits. The other side said that you pay tax as ordinary income on the FMV of the coin at the time you gain possession of the said coin.

2019 IRS Crypto FAQ

In 2019, the IRS issued guidance on how to treat airdrop tokens. On Q23, the IRS established the fact that you must report the FMV of the tokens when received as ordinary income, and that becomes the cost basis. So if the token was valued at $100 when received in 2020, you would have to pay tax on the FMV as ordinary income. Say it dropped in price in 2021 to $50 and you sold it. You could claim that as a short-term capital loss in 2021.

In short, any time that you receive a legit airdropped coin, you need to pay taxes on the value of that coin when received.