In ArcCatalog, a pure PostgreSQL database table created in the SDE schema cannot display the Allow Null Values parameter as Yes.
Last Published: May 26, 2016ArcSDE/Enterprise Geodatabase
Bug ID Number
January 23, 2015
June 11, 2020
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
This is a limitation of the libpq library. The libpq library does not make available information about the nullability of a column in a query describe so that’s why information is not appearing as it does for the other databases.