The JSON API Extras module allows overwriting endpoints automatically exposed by the JSON API module. This allows disabling fields from being returned. It also allows using enhancers to simplify the structure of a field property.

From the administrative toolbar, go to Configuration. Under the Web services section, click on JSON API Overwrites to customize the JSON API implementation.

To disable an endpoint, click on Overwrite on any endpoint. Check the Disabled checkbox to turn off that specific endpoint:

To disable, alias, or use an enhancer, click on Overwrite on any endpoint. The checkbox will allow you to prevent a field from being used in the API. The enhancers allow you to simplify fields when returned or used in POST/PATCH requests:

In this example, the created and changed fields will no longer return Unix timestamps, but RFC ISO8601-formatted timestamps. The promote and sticky fields will return their value directly, not nested under a value property. Finally, no revision information fields will be returned.