The delivery option for forwarding isn't available on the XML API.
However it is available for usage with the CLI, which you can run via the REST API with the CLI method.
Code:
plesk bin mail -u <[email protected]> -forwarding true -delivery false
As a side note: Unfortunately the -delivery option for the mail utility (still) isn't listed on the CLI documentation page (3 years after it got introduced). But it got mentioned on the Change Log for Plesk for the 18.0.42 release.