Continuing the discussion from XmlMappingType.Hidden ignored for JSON output in API:
As hiding the data field in API is the primary use case for using XmlMappingType.Hidden in entity/data structure fields, let’s move it out of the XmlMappingType as it is also related to other formats, like JSON.
I suggest to create a boolean attribute, e.g. HideInOutput and migrate all entity/data structure fields that were set to Hidden to true.
Then we should remove the Hidden from the XmlMappingType enumeration.
The new functionality should remain basically the same – in case of XML it would hide the field for XSLT transformations or APIs and it would also hide the field for JSON output in APIs.