Make it possible to store NaN values for Z values. This is possible for M values.
If this is possible for Z values, there can be made a difference between a Z value 0 and a Z value which is unknown.
The documentation says a Z value can be set to NaN but this isnt't true.
https://pro.arcgis.com/en/pro-app/latest/help/editing/edit-vertex-z-values.htm
See also BUG-000152196 Clearing a z-value results in zero, instead of NaN