That privacy-protecting screenshot detection "feature" sounds like DRM. Android will likely prevent screenshots on websites and content that requests it.
(android can already tell if you take a screenshot...)
That isn't the purpose of the new API. Apps can already prevent screenshots by using FLAG_SECURE.
The purpose is to move away from the resource- and privacy-invasive ways that apps implement screenshot detection today, such as through monitoring the directory in which screenshots are saved. The new functionality will require a permission and will no longer require the apps to have access to the screenshots themselves.