VDU 21 disables screen output. It is reversed with VDU 6, and the state can be read with OS_Byte 117.
Characters are still sent to the printer (if it has been enabled with VDU 2) and any other ouput devices enabled with OS_Byte 3. Multibyte VDU sequences are still passed as usual (but then ignored if they affect the screen) - hence sending 22, 6 while the screen ouput is disabled will neither change the mode (VDU 22) nor enable screen output (the 6 byte is parsed as the parameter to the ignored VDU 22).
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