We have a large number of published assets on the Brand Portal that need to be removed. It's a little difficult to find in Author (not impossible, but will require some reporting/queries). I am wondering what happens to their status in Author, if I delete the images from the Brand Portal?
Directly deleting images from AEM/Dynamic Media/Brand Portal isn't recommended.
You need to unpublish/deactivate the assets first and then delete them from the system.
Reason: The assets if removed in published state, will still be cached for a specified TTL (~11hrs). This will bring inconsistencies to your website where the images are displayed (some images might disappear, some will still show up and no way to track and fix the behavior).