Hi @wy-2 ,
Marketo moved their open-beta Self Service Flow Step (SSFS) feature to GA in the last product release. The SSFS essentially allows you to install compatible custom third party web-services as the custom flow steps that can be used in the smart campaigns’ flow. Any data updated by SSFS is marked as being sourced from the Custom Flow Action in the Marketo’s activity log. Like other input sources you can now choose to block field updates on the fields from the SSFS in the field management section as well.
You should block updates from Custom Flow Action on your one-time write fields and the fields that you don’t want to be updated by SSFS from the Field Management section.
Read all about SSFS here - https://experienceleague.adobe.com/docs/marketo/using/product-docs/core-marketo-concepts/smart-campaigns/flow-actions/flow-step-service.html?lang=en