All US citizens (by this we refer to US citizens AND US resident) are subject to mandatory KYC according to the new guidelines. If you are a US citizen or resident, you will not be able to trade FIAT until FATCA reporting is fully implemented. In the meantime, you will still be able to trade Crypto to Crypto on COSS.IO. And COSS will continue onboarding the new users from the US.
Once FATCA is in place the option to trade FIAT will be open to you, and no new submission of documents will be required.