Investor’s ability to trade U.S. government debt has deteriorated to its lowest point since the ructions of March 2020, deepening worries about the world’s most important bond market as the Federal Reserve tightens the monetary policy. Liquidity, or the ease of buying and selling, in U.S government securities has dropped since the beginning of this year, reaching levels not seen since the first months of the coronavirus crisis, according to an index compiled by Bloomberg.