I was told by a smart contract developer that, due to contract chaining (or subcalls), the problem of knowing if an address in a transaction has interacted with Tornado, is equivalent to the halting problem, because one can craft a contract that behaves differently when being checked than when being run, by checking chain state data.
Seems like these protocols used a centralized service that provides information about which addresses are subject to OFAC and they just blindly used the results.
Reminds me of records and record players...