In the US, it is (unfortunately) still common to use US survey foot. It seems at least some DXF applications/libraries support this unit as a $INSUNITS value, some references:

• https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2020/ENU/AutoCAD-Core/files/GUID-A58A87BB-482B-4042-A00A-EEF55A2B4FD8-htm.html
• https://ezdxf.readthedocs.io/en/stable/concepts/units.html#insunits

There are two issues regarding this with QCAD:

1. If you open a document with $INSUNITS being 21, it's reset to 0 (unitless/unspecified) when saved again, even when not changing anything.
2. It is not possible to select these units in the GUI.

I don't know about standardization of these values, but my hope is that it's at least possible to preserve the unit in an existing drawing. Given that this unit is supposed to be obsolete in some years, it might seem less important, but due to this transition, it might get even more important to be able to distinguish between units. Thanks!