Regulation S requires US persons no matter where they live to only trade at SEC regulated exchanges. [1] So broadly speaking it applies to all US persons - residents and citizens regardless of place of residency.

I suspect accredited investors can do as they please though, but that's only a guess.

[1] https://www.law.cornell.edu/cfr/text/17/230.903

> they are illegal only on regulated markets

How on earth would your take-away from that be that be "oh, in that case it's perfectly legal on all the _unregulated_ markets" ?

That not how regulation works, at all.

It's not illegal for hedge funds.

I believe Archegos bought total return swaps. Those are different from CFDs, but I'm not sure how different they are. I'm not sure why one would be illegal (like CFDs are) and one would be legal (like total return swaps).