I have just upgraded one of my sites, not the Prod site, the Local one. using a script I have created which contains the command below:
drush dlb backdrop --2020-05-21 17:45:47-- https://github.com/backdrop/backdrop/releases/download/1.15.3/backdrop.zip
As you can see it downloads version 1.15.3 which is the PREVIOUS Version!!
Downloading from the Link in the Available Updates Page all is well, so the link using Drush must broken.
When I upgraded to 1.16.0 a week or so ago using my script I had no problems.
Regards to the Team.