In design field properties, domains only show if they have been assigned on those field before subtype functionality is enabled on a Feature Class. After subtype is set on a field, they don't show. I suspect that it is because domain could vary from subtype to subtype, therefore design field properties present domain information as blank value, which is misguiding.
I would recommend enhancements for design field properties as follow :
A - If all subtypes have the same domain assigned to a field then show the “Domain Name”
B - If various domains are assigned to the field depending on subtype used, then show “Various Domains”
C - Same as A & B for Default Values
In addition, I would recommend a columns in the design field properties to show if Attribute Rules is/are set/enable/disabled on a field. It would be rather convenient to access a functionality from design field properties where one could access the Attribute Rule definition(s), modify the Attribute Rule and set the Attribute Rule Order.