I think that belief is mistaken. (1) Suppose Bitcoin has a contentious fork into two chains, say BitcoinA and BitcoinB. The supporters of each chain claim that their chain is the "true" Bitcoin. Before the split, you had 1 btc worth $10k, and after the fork, you have 1 btcA worth $5k and 1 btcB worth $5k. Which of the two chains do you owe taxes on? (2) Even if a hard-forked chain is treated as a new asset instead of a split of an old asset, it shouldn't be taxed until the user actually exercises control over it. In order to control funds on a new hard fork, generally the user must take the risky action of installing new (potentially malicious) software and importing their private keys --- this would seem to be a "substantial limitation or restriction" under 26 CFR 1.451-2(a).