M84

Some users wonder that

M84 X Y E

does not work with the firmware.

You probably missed this part in the installation.txt and the end gcode:

;M84 ;Disable all steppers. It is not possible to disable only X Y E cause of the shared pins
; M84 is deactivated by default. As we can only deactivate all steppers there is a chance that
; z crashes into the print when the steppers are deactivated on print end.
; this you should decide yourself!

The stock firmware is based on Marlin 2.0.8 which handled this a bit different. Although the pins are shared one could send M84 X Y E and it worked.

Marlin 2.0.8 does not handle such a call so fine like Marlin 2.1.x which is used for the fork.

This means that Marlin 2.1.x detects that the pins are shared and so "M84 X Y E" is impossible to execute. Marlin 2.0.8 simply disabled the first found pin and disabled with this all shared enable pins and continued. Marlin 2.1.x stops here and does not execute the "impossible" command.

M84 

without anything is the right way to do the same what was possible with the stock firmware.