I recently tried to exclude a folder that was once included in a pre-existing mirror backup job that is set to "Remove excluded or deleted files from backup". After re-saving the backup job with the newly added exclusion and running the backup, the folder is NOT being deleted from the existing backup. The new exclusion does "appear" to be being saved properly in the backup job.
FBackup4 Version 4.8, Build 486
I ended up deleting the folder that I wanted excluded, manually, from the backup. Then I re-ran the backup. More than once. So, I doubt this would be useful.