Thanks for the output.
Unfortunately, the sources.list shown is accessing two releases, jessie and bullseye, which, in debian parlance, is called a "FrankenDebian". On the relevant website:
wiki.debian.org
there is the fairly clear statement:
There's lots of other info there which is probably worth reading.
On the matter of the package: steam-installer, it is in fact in the bookworm release which one can check at the debian packages website here:
Upon entering a search for "steam-installer", the returned output is:
Code:
Exact hits
Package steam-installer
bookworm (stable) (games): Valve's Steam digital software delivery system [contrib]
1:1.0.0.75+ds-6: amd64
I guess you have to make some choices about which release you wish to run to achieve what you'd like to have functioning. It looks like steam may benefit from bookworm, so you may consider an upgrade.