Some platforms require KYC even for signing up and trading crypto to crypto, but from experience, it's fairly easy to trade crypto to crypto, there are a number of platforms what will let you do so, or you could always VPN and use services that are closed for US IPs but still accept VPN's (like a couple mentioned in this thread.).
Yes, I was asking as for the legality side of it, if in the US it is required for the platforms to enable KYC even for crypto-crypto only platforms