Oh wait looks like it was working in Blender 2.8 but then disabled for Metric and Imperial units since 2.81.
if you change the units to “None” supposedly it will stop being greyed out (but only while in orthographic view). There is commentary on the dev 👾 pages:
https://developer.blender.org/T70764
I guess it messes up distance units and that’s why it is turned off when they are active.
