Definitely understand the pain of having to rely on headless browsers for such tasks!
On this one, we don’t expect the UPM backend to be decommissioned any time soon, so the REST API should remain available even after the change in this post.
Moreover, as we’ll still need an API for the admin.atlassian.com frontend even if the UPM backend were to disappear at some point in the future, I see no reason for us not to continue to provide access to the direct API call for development purposes.