Considering how we currently use fonts, it does seem odd that the source files are bundled with the game. I guess they're only there because they're in the "binaries" folder, and that folder is bundled almost in its entirety.
As a loose idea as to how many distributions have the fonts available in their package repositories (stats courtesy of Repology):
LinuxLibertine : 66 packages (across 20 "families")
DejaVu : 145 packages (across 33 "families")
FreeFont : 122 packages (across 26 "families")
TeX Gyre : 37 packages (across two package names across 5 "families")
And for the eastern Asian languages:
Hanazono : 48 packages (across 10 "families")
Japanese Kanji
Source-Han-Sans : 92 packages (across 13 "families")
Pan-CJK characters
(Repology groups distributions by "family", e.g. Ubuntu, Debian, SteamOS, LinuxMint are one "family". If you're running LinuxMint, then as LinuxMint is derived from Ubuntu, then the repositories for the appropriate version of Ubuntu are mostly compatible with your LinuxMint system. It's not a perfect categorisation, but as a rough guide it works.)
Alternatively, here's something I threw together to more easily compare some of the commonly used distributions. CentOS, Fedora, OpenSUSE, OpenMandriva, and Slackware are the ones that are noticeably problematic.
Yes, Windows and OSX would most likely still need the fonts bundled with the game.
The way I see it, we have three possibilities:
We change nothing, but tell the packagers to exclude the "binaries/tools/fontbuilder" when packaging the "0ad-data" package for their repositories.
We remove the font files entirely, make the files' respective packages a build-time-only dependency, and write instructions on where to download the files for systems that don't have a package repository or do but lack the specific packages. (The caveat is that we don't use them at build-time either...)
We relocate the font files somewhere else within our repository, such as into a subdirectory under "libraries/source/".
I think I personally favour a combination of the second and third: move the files and then, when we get in-game rendering working we add package dependencies and use the new location as a fallback (whether that be as the source of a 0ad-fonts package, auto-bundling of the fonts by build-osx-bundle.sh, and/or some other solution).
I think the fonts we use are reasonably stable (LinuxLibertine and FreeFont haven't had any releases since 2012, and DejaVu since 2016. TeX Gyre is the most recently updated at 2018, but the release before that appears to have been 2016).
I would imagine that if the typeface anatomy of fonts change too much between versions, then the font's creators would possibly get complaints from their user-base. Also, some of the fonts we use claim to be a like-for-like replacement (Gyre Pagella for Palatino), or to have the same appearance as - but a wider character range than - another font family (DejaVu for Vera), and so in theory shouldn't deviate too much from their "inspiration".
I get what you mean though: having two users with what should be the same font but - due to changes between versions - are visually different could disrupt our established visual style (such as it is), or make for an inconsistent experience between users.
It is true that many games get bundled with their own fonts, even on Linux systems (eg. SuperTux, SuperTuxKart).That might be down to simplicity: they have to bundle the fonts for Windows and OSX systems, so it's easiest to just do the same for Linux as well.
Huh, this turned into a longer post than I anticipated. Apologies, and congratulations if you got this far without glazing over.