Could it be because it’s EOL and finally got around to removing it to archive?
as you may know, oldstable is only supported for 1 year. For bookworm this was 2024-06-10. We added a small grace period afterwards, but we will not allow updates after this point. We will also remove the suite from the debian mirrors soon.
Thanks for your attention
Originally posted on debian-backports-announce
Posted Tue Jul 2 11:44:15 2024
Also. https://www.reddit.com/r/debian/comments/1m6gpaf/strange_problem_with_bullseye_backports/