In general, the Data Catalog export file only contains the metadata information of the Virtual DataPort Server registered in the Data Catalog.
Since the metadata is only exported during the export process, the actual value provided in the variable “$user_name” will not be exported to the exported file (custom_property.json). This is because the variable defined in the property group would be updated only at run time in the Data Catalog web tool.
For example, if a user1 is logged into the Data Catalog, then the $user_name variable will be shown as user1 and when user2 is logged in then the $user_name variable will be shown as user2. So it is not possible to export the metadata with the actual data as the value is varying based on the logged in user.
To answer the question on getting “Last modifier” and “Last modification” – Currently,the option to verify the last modification date is only available for the views(localviews.json) and not for the other metadata objects such as properties, tags, categories etc..
For additional information, you could refer to the [Manage the Custom Properties of a Property Group](https://community.denodo.com/docs/html/browse/8.0/en/vdp/data_catalog/administration/property_groups_management/property_groups_management#manage-the-custom-properties-of-a-property-group) of the Data Catalog Guide.
Hope this Helps!