With everything slowly( or sometimes rapidly) moving into containers (docker, systemd portable services, flatpak, snaps) I think the concept of system library will probably become irrelevant at some point not that far into the future.
I don't think we're anywhere near a future where "ls" will be run in a container.
I used to think that we're not anywhere near a future where most applications came bundled with an entire browser, yet here we are.
It's still a system library just a system stuffed into a container.
Containers (and containerish systems like flatpak) still have portable dependencies that are equivalent to "system" libraries in this context.
This would make it impossible to achieve the most basic level of security.
How does it matter whether a library is a "system" one or not?
The expectation is that with a "system" package, one can update that one package and (basically) everything on the system now uses that new version. Practical for security and important bugfixes.