Most of the money banks “have” they created it themselves (it’s called Fractional Reserve Lending and there’s a wonderful paper on it by somebody at the Bank Of England called “Money creation in the Modern Economy”).
The whole “banks lend out depositors’ money” thing hasn’t been true since the 80s.
Also nowadays most of the money in leveraged investments comes from the Money Markets (so rich people and pension funds) rather than banks.
That said, your point still stands since a reduction of deposits might impact banks’ reserves (basically central banks force them to have the equivalent of about 3-5% of their loans as reserves), it would force a wider retrenchment of their loans, but by itself the impact of that on the entire leveraged investment universe should be limited because they’re not the main players in the Money Markets.