Based on the feedback I received from Support, it's not possible to automate GDPR removal requests since there's no API endpoint.
We need to manually delete one by one using the web interface "Delete This Contact" button, which calls:
https://cloud.maropost.com/accounts/####/contacts/######/pseudonymize_contact
We process 20 removal requests per week. Automating would save time on our side.
Please make this function available throught the REST API.