I now understand that you prefer PureOS because it's "Pure."
I was surprised that you were interested in flatpak – on the one hand, "Pure" and on the other, flatpak – but let's leave that aside.
Just because Pure hasn't released an updated ISO image for two and a half years doesn't mean they're not doing anything.
I looked at their repositories: https://repo.pureos.net/pureos/pool/main/
You can see that they're releasing newer packages there:
https://repo.pureos.net/pureos/pool/main/b/base-files/
There's also 0ad:
https://repo.pureos.net/pureos/pool/main/0/0ad/
Therefore, I suspect you didn't read the PureOS documentation about how to update releases.
PureOS is based on Debian, and I suspect it also uses Debian release codenames. You're probably stuck on Bullseye – hence the version 0.23.0 available.
https://packages.debian.org/bullseye/0ad
I would try this:
I would check the entries in the '/etc/apt/sources.list' file – if the word 'bullseye' is there, that's the cause.
In that case, you should update sequentially: bullseye -> bookworm -> trixie, rather than straight to trixie.
But to be sure, I would read the PureOS documentation.
If this update were successful, I would download three files: 0ad, 0ad-data, and 0ad-data-common (version 0.27.1-3 from Debian trixie-backports):
https://packages.debian.org/trixie-backports/0ad
and I would install them manually (dpkg -i file.deb).
I would then have the latest 0ad for PureOS – but this is just my guess, as I've never used PureOS.
In summary – first, issue the following commands:
cat /etc/os-release
cat /etc/apt/sources.list
Regarding your question, I believe there are currently no Linux distributions that meet Richard Stallman's rules.
Which distribution doesn't use proprietary firmware?
Best regards
Postscriptum
---------------
PureOS does not use Debian's code names, but its own: Amber, Byzantium, Crimson, and Dawn.
https://repo.pureos.net/pureos/dists/
Dawn is probably the equivalent of Debian's sid (perpetually unstable).
So you need to read up on it.
My guess: you have Amber, but you should have Crimson.
Check it out.