As someone that helps build an operating system distribution, we consider Xscreensaver to be a system component.

Also, for licensing purposes, the GPL, as an example, considers software distributed with the system to be a system component in some scenarios. For example, "GPLv3 has adjusted the definition of System Library to include software that may not come directly with the operating system, but that all users of the software can reasonably be expected to have."

xscreensaver is X11, not GPL.

Why does a C++ ABI really matter in a system distribution? All the distros I've used are internally consistent in their ABI and update all their deps when the C++ ABI is updated.

Not from a distributor's perspective ;-)

xscreensaver is X11, not GPL.

Correct, but since it is distributed as part of the system, or is a component that most users will have, then, as an example, if you link your GPL'd code to Xscreensaver's API, that's ok, because it's considered a "system library".

Really, I was referring to more for the purposes of definition than anything.

Why does a C++ ABI really matter in a system distribution?

Because ABIs create a point of synchronization, and internally for that distributor, that create a flag day where a given set of components must be rebuilt at the same time.

Ideally, for a distributor, you don't rebuild the entire world every time one component changes. You only rebuild what you have to, since every delivery causes new binaries to be produced, which then in turn increases the amount of downtime for consumers that are upgrading.

As such, changing ABIs (thankfully GCC's doesn't change as much as it used to) create unnecessary churn, and if you can easily avoid them, you usually do (as a distributor).