There should certainly be more tooling around finding these things out, but one thing worth noting is that you can `flatpak uninstall --unused` to remove old, unused runtimes (the runtime environments for apps).
Is my system is guaranteed to fill up completely if I don't run that command regularly?
It could also benefit users that have more disk space than bandwidth (I was in that situation for a while, though a mechanism for sharing runtimes with LAN peers would be nice).
Discussion: https://github.com/flatpak/flatpak/issues/2639
Related blog entry: https://blogs.gnome.org/mwleeds/2021/01/11/cleaning-up-unuse...
Remove EOL automatically: https://github.com/flatpak/flatpak/pull/3871