With Debian, one can apt-pin different releases on as well. So you can run testing for example but have oldstable, stable, unstable and experimental all pinned on.

That maximizes your chance of being able to satisfy a particular dependency like libflac.8.so. Sometimes that might not actually be practical to pull in or might involve massively changing a lot of your installed software to satisfy the dependencies, but often it can be a quick easy way to drop in more libraries.

Sometimes libraries don't have a version number on them, so it'll keep being libflac even across major versions. Thats prohibitive because ideally you want to install old version 8 alongside newer version 12. But generally Debian is pretty good about allowing multiple major versions of packages. Here for example is libflac12, on stable and unstable both. https://packages.debian.org/search?keywords=libflac12