A tax loss on Tether (USDT) would occur if you sold the USDT at a lower price than what you initially paid for it, resulting in a loss. This loss can be used to offset any capital gains you may have realized during the tax year.
For example, if you purchased 1,000 USDT for $1 each, resulting in a total investment of $1,000, and later sold those USDT for $0.90 each, you would have received $900, resulting in a $100 loss.
If the net amount of all your gains and losses is a loss, you can report the loss on schedule D of form 1040 up to $3,000. Losses beyond that amount can be deducted on future returns as a capital loss carryover until the loss is all used up.