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).
Agree with the previous answer that unpublish is better, but since you're only using the Brand Portal rather than AEM Sites, the impacts of straight deletion are less serious. (The TTL situation I believe only applies to AEM Sites with a dispatcher and/or CDN caching layer in front of it).
Basically, the only impact of a straight delete on the Brand Portal is that the author instance (i.e the DAM) will still show those assets as published to the Brand Portal. The deletion on the Brand Portal just removes the assets--it doesn't send any message back to the DAM to adjust status / metadata there.