Looks like they've given up.
https://blogs.imf.org/2019/02/05/cashing-in-how-to-make-negative-interest-rates-work/
In a cashless world, there would be no lower bound on interest rates. A central bank could reduce the policy rate from, say, 2 percent to minus 4 percent to counter a severe recession. The interest rate cut would transmit to bank deposits, loans, and bonds. Without cash, depositors would have to pay the negative interest rate to keep their money with the bank, making consumption and investment more attractive. This would jolt lending, boost demand, and stimulate the economy.
When cash is available, however, cutting rates significantly into negative territory becomes impossible. Cash has the same purchasing power as bank deposits, but at zero nominal interest. Moreover, it can be obtained in unlimited quantities in exchange for bank money. Therefore, instead of paying negative interest, one can simply hold cash at zero interest. Cash is a free option on zero interest, and acts as an interest rate floor.
https://blogs.imf.org/2019/02/05/cashing-in-how-to-make-negative-interest-rates-work/
