Now I think I'm getting somewhere. In the output posted above, I see a lot of references to a new line I recently added to /etc/apt/sources.list as I had read in some instructions for installing restic (which didn't install successfully so far). The offending line seems to be:
Code:
deb http://ftp.us.debian.org/debian buster main
Now I recognize buster as a nickname for a more recent version of Debian. If I comment out that line, Software Update no longer recommends updating all those packages; it says there are no software updates available for my computer at this time. Also the output from apt-get update is a bit shorter.
Now I will mark this question as solved, and look into the version upgrade in the near future.
Thanks ondoho.