The Confluence Cloud team has created new permission API to grant or restrict custom content type permissions access for individual users and/or groups. This will help apps that use custom content for their permissions model to restrict specific users and/or groups from gaining access.
Note:
This new API can only be accessible by Connect apps.
Connect app needs to have WRITE scope to access this API.
If I remember correctly, this was one of the Confluence APIs that existed for a long time but simply were not accessible by Connect apps, as is currently still the case for other APIs such as some Group and Audit Log ones. Do you know if there’s any chance of those APIs being allowed to be used by Connect apps any time soon as well?