Only unused apps / app versions can be deleted.
If you delete an app / app version, it will be immediately removed from the UMS, i.e. without moving to the recycle bin.
Tip: If all objects that use an app seem to be removed, but it is impossible to delete the app since the system declares it as used, check the recycle bin for devices and profiles that can still use the app and delete them. For more information on the recycle bin, see Recycle Bin - Deleting Objects in the IGEL UMS.