Like torrents or dark markets. Technically not a problem, but will cause more effort and risks for creators (goodbye VC backed "Decentralized" swaps) and higher costs for clients who will pay for covering those risks.

I assure you, though, that some sneaky guy from Africa, Russia, Ukraine or China would always find his way to help people changing their money.

Btw, in theory there is a way to create completely decentralized versions (hosting frontend on IPFS/torrents/other distributed storage) and the whole backend made on smart-contract, but it would be slower and harder to use than current systems with partially centralized backends and CDN js/static distribution.

Can TCP/IP still work if it's outlawed?