the global financial system depends on the abundance of this collateral, otherwise - defaults, margin calls, etc i wrote a thread/article explaining how US Treasuries are the dominant collateral in short-term wholesale debt markets (e.g. repo). read here: https://illya.sh/threads/@1751726431-1.html
QE also removes safe collateral from the market, mainly US Treasury bills, notes and bonds. this safe collateral is the backbone of wholesale debt markets, where financial institutions, including commercial and central banks finance and re-finance their positions