When a user creates a feature class (line or polygon), shape fields are automatically generated. When editing, those fields are also automatically updated based on their 2D features (length or area) and coordinate system.
When ArcGIS Pro was released, all new feature classes were generated with z-values per default. Yet, Esri hasn't included 3D shape fields until now!
The work around is to add a new field and run a geoprocessing tool "Calculate Geometry". This is not feasible in an editing workflow, f.ex. in the field when updated 3D lenghts and area are needed for verification, calculations and other processes. Another work around is using Arcade and attribute rules. Again, only work arounds and no solution to this issue is in sight according to Esri.
Using ArcGIS Pro with z-aware feature classes for many years now, Esri HAS to address this issue and make their features fully z-aware without cumbersome work arounds! Ideally, a new shape field is added to all z-aware feature classes upon generation and those two fields live side by side in the attribute table: point z for point feature classes, shape_length and shape_length_3D for line feature classes and shape_area and shape_area_3D for polygon feature classes respectively.
Therefore, the more users report their use cases, the sooner Esri will start working on this (understandably) comprehensive issue.