Ask the Debian maintainers. That was their recommendation, and I trusted them - presuming they would recommend something that would work more than two weeks on a rather standard laptop installation. I will have to re-partition within the next year, because their / partition is too small as well.
I think this happens because the default option is “recommended for new users”. So some not-new users believe that the other options are better for them.
That default options reads like this:
“All files in one partition (recommended for new users)”
No, they make more than one recommendation - including which partitions to make and the sizes for each of them should you opt into their separate partition path in the installer. So they have defaults for multiple partitions and partition sizes - and I trusted them to have thought them through.
Two improvements that could be made: 1) Easy: put a brief Note in the installer indicating what might fill up the partitions quickly so people can have a heads-up, do a little research, and make a better decision. 2) moderate: still keep the Note, but also check the disk size and maybe ask which type of workload (server, development, home user), then propose something a bit more tailored.
Because it isn't used for much? It's mostly just logs these days. Most data on most systems goes in /usr or /home. I would say the weird thing here is that Flatpak puts runtimes in /var by default instead of ~/.cache or something like that.
User-mode Flatpaks keep things in ~/.local/share/flatpak. This person simply installed a Flatpak in system-mode, which puts it somewhere other users could also run it (i.e. not your home directory).
