It depends. For example, stronghold USD requires Auth to trade on StellarX.
https://www.stellarx.com/markets/USD:GBSTRUSD7IRX73RQZBL3RQUH6KS3O4NYFY3QCALDLZD77XMZOPWAVTUK
The website link provided is from the asset's verified stellar.toml web address. Most issuers use a third party KYC service.
If the asset is not auth_required, then it is freely tradable via the SDEx. The issuer may have performed the ICO/STO using a closed, KYC compliant method other than SDEx.