What is "tainted" USDT
Tainted USDT usually means funds linked to scams, theft or money laundering. Tether, the issuer of USDT, can add an address to the USDT contract blacklist. A blacklisted address can no longer send USDT, so its funds are effectively frozen.
Beyond on-chain freezes, receiving tainted funds can also lead to bank accounts being frozen in some jurisdictions. OTC desks and payment providers are the most commonly affected.