Andrew,

I know that GCodeMM.js and GCodeIN.js are generic postprocessors.
Mostly used as base for other or custom postprocessor.

The distinct nature is pretty clear from the filename and the suffix in the display name.
Another distinct nature is that they include this.unit = RS.Millimeter / RS.Inch intended for the CAM side.

Why don't they include at least a G20/21 in a generic header?

There are several more specific postprocessors where I miss these G-codes for the measurement system.

True, if they are not supported, no error messages would occur when omitted.
The danger is that users rely on build in postprocessors hoping that it matches with the default measurement system of their setup.

IMHO it is safer to trigger an instruction fault before attempting to execute a wrong motion.
As fail-save as possible should always be the goal.

Regards,
CVH