If you (or someone else) still need to use systems based on Buster (debian 10) for a little while longer I recommend to add the elts support repositories for additional security updates (although limited).
How to use Extended LTS
Adding extended LTS repositories to APT Installing the freexian archive GPG key The extended LTS repositories are signed with the following GPG key:
sec…
www.freexian.com
About repository archived that give error like backports of the first message, need to change the url to archive:
Code
E: The repository 'http://httpredir.debian.org/debian buster-backports Release' no longer has a Release file.
I mean change http://httpredir.debian.org/debian to http://archive.debian.org/debian/ in the backports line of your apt source.list