I'm not exactly sure when G20 and G21 were removed from the Marlin firmware but they work in 18.104.22.168 and are unknown in 22.214.171.124 and 126.96.36.199.
I can't comment on the removal of the commands but if they were removed on purpose, they should also be removed from the Start Gcode.
In CuraLE 3.2.32 the TAZ 6 Single Extruder start gcode still has a G21 in it. I have not (yet) checked all of my other extruder start gcodes.
UPDATE: FW does not support these commands, remove from all start gcodes.