I finally got it fixed.
I had to remove the apt.llvm.org repository from the sources.list. Then I could use the Force Version command in Synaptic to switch over to using LLVM builds from the kisak PPA (which still provides i386 builds of LLVM). That enabled me to reinstall steam, because it could finally get those 32-bit i386 library builds again.
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Then I tried to do it. Error after error.
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old MacBook from 2009
There is a huge number of tutorials available online for ...
If you're comfortable with the command line, you can use the dpkg command to install a .deb package.
The format would be sudo dpkg -i name_of_package.deb
then, sudo apt-get install -f. the dpkg -i command may leaves the package status in an "unconfigured" state due to missing dependencies.
sudo apt-get install -f will try to fix it downloading and ...