Where can I find the US Federal Reserve Margin Supplemental "Free Riding" Rules?

Yes, IBKR Pro and IBKR Lite margin accounts are subject to the PDT rule even if you currently do not hold a margin loan.

You can find an Overview of Pattern Day Trading ("PDT") Rules on the IBKR website…

You can find the FINRA Margin Rule 4210 on the FINRA website…

You can find the US Federal Reserve’s Credit By Brokers and Dealers rule (Regulation T) on the electronic Code of Federal Regulations website…

Cash accounts maintained with IBUK are subject to the U.S. Free Riding Rule as the accounts are introduced to and carried by IBL, a U.S. broker. According to the Free Riding Rule, in a Cash account, an investor must pay for the purchase of a security (meaning, the trade must settle) prior to selling that security. If an investor buys a security and then sells that same security without paying for the security in full by settlement date, the investor is considered to be "free riding." Accounts that commit free riding violations will be restricted for 90 days, during which time the account can only purchase securities using settled funds…