ArcGIS Pro - Preset Subtype Group Layer - Design Subtypes Properties
I see what you're saying, that Design > Subtypes is grayed out:
But my question would be why do you need to re-design the subtypes for a subtype group layer? It seems to me that changes made to the feature class' subtypes would nullify the existing subtype group layer and that it would have to be removed and added again.
With an existing subtype group layer in my map, I went to the feature class in the Catalog Pane > Design > Subtypes (which is how you are able to still access and modify subtypes for the feature class that is the source of the subtype group layer) and when I modify it, I see that it does "nullify" the existing subtype group layer. I can re-add it to my map and all is well.
Do you have a workflow that requires you to have a subtype group layer in a map while you're modifying the subtype definition? I guess the request is to have the subtype group layer be dynamic, right? And update automatically when the subtype is modified?
I wonder if there are other users out there that want the opposite; i.e. leave the subtype group layer alone and let me modify the subtypes of the source feature class.