Even before the Fed removed the reserve limit altogether, banks would frequently hold less than the minimum reserve ratio, and simply borrow from each other or borrow from the Fed to meet the requirements. Essentially, as long as the bank was willing to pay a fee (the interest on the borrowing), they could hold any amount in reserve they wished, and create and loan out any amount they wished. The fractional reserve requirement is a way for the Fed to influence liquidity and interest rates, and not a way for the Fed to place a limit on how much new money a bank is able to create and loan out.

It's not really clear how this would work with bitcoin. If everyone started trusting centralized third parties to hold their bitcoin, then these third parties will only be limited by the volume of withdrawals or transactions they have to process each day, which will always be a tiny fractional of the total amount being held. If they start conducting off-chain settlements with each other similar to how fiat banks do business with each other (for example, Coinbase and Binance keeping a running total of how much they owe each other without ever actually transacting any coins), then it just becomes farcical.