structures/hellenes/fortress_up.xml and structures/spartans/fortress_up.xml could probably be deleted as well, since they appear to be unused and look the same as the Athenian and Macedonian fortresses.
Also, why is there a special/old/ folder?

In rP23351#40604, @Nescio wrote:

structures/hellenes/fortress_up.xml and structures/spartans/fortress_up.xml could probably be deleted as well, since they appear to be unused and look the same as the Athenian and Macedonian fortresses.
Also, why is there a special/old/ folder?

Ideally all unused old buildings and other things should go in the art_source repository, and be put in the old public directory. But a lot of modders wanted to keep access to old structure without having to bother with that, so instead they wanted us to store them somewhere. That's a remnant.

Wouldn't it be better to move all that to a separate mod instead?

For me yes. We shouldn't force people to download more assets than they need whether it's in SVN or in a release. Some of our users still pay internet in MB, and I believe it's nice to be considerate.

For people like wow though, it's more convenient if leave unused stuff in the game. Did you know for instance that my Colosseum wonder was committed? I didn't but it's included since rP19095

Unused stuff that's up to standard (e.g. carthaginians/palace.xml) is perfectly fine, but unused stuff that's been replaced years ago (e.g. what's inside special/old/) is not worth keeping. I doubt those assets are still being used by any active mods, and if they are, setting a 0ad_old mod dependency shouldn't be too difficult.
Besides, when other assets were updated (e.g. @Alexandermb's horses), the old versions weren't kept separately, were they? If someone really wanted those, they could find them in the svn revision history.

In rP23351#40610, @Nescio wrote:

Unused stuff that's up to standard (e.g. carthaginians/palace.xml) is perfectly fine, but unused stuff that's been replaced years ago (e.g. what's inside special/old/) is not worth keeping. I doubt those assets are still being used by any active mods, and if they are, setting a 0ad_old mod dependency shouldn't be too difficult.
Besides, when other assets were updated (e.g. @Alexandermb's horses), the old versions weren't kept separately, were they? If someone really wanted those, they could find them in the svn revision history.

There also unused variants (frequency="0") and object that are dangling (no reference) some of them can be found with checkrefs.pl --check-unused.

(Can't really say whether artwork is unused or used if you account for the fact that a new scenario map using it could appear in a mod or repository every day if the quality of the artwork is fine.
Then it only needs to be decided whether it fits to the 0ad frame or not, and if so, it should be sorted logically I suppose. "old" should then be emptied by moving or deleting?
For example when I create new maps, I browse through the entire list of actors and filter out what I see might fit to the map and prefer unused actors, because that gives the map uniqueness.
That doesn't warrant keeping bad quality art however. I didn't look at this specific actor(s), so Im not sure if my 2 cents are actionable.)

Unused art that's up to standard is fine, I actually like that. However, what's inside special/old/ has been deprecated years ago and is visibly of inferior quality, so I was wondering why they're still kept. Just an example:

In rP23351#40664, @Nescio wrote:

Unused art that's up to standard is fine, I actually like that. However, what's inside special/old/ has been deprecated years ago and is visibly of inferior quality, so I was wondering why they're still kept. Just an example:

From left to right:
structures/athenians/theater.xml
special/old/hellenes_theatre.xml
special/old/hellenes_theatre_2.xml

Mostly because there hasn't been much cleanup before I joined the team. One of the big goals I have is to go through all the unused stuff, find false positives, improve what can be, create templates for those, and nuke (or move) the rest. This is quite a boring task however, so it's slow :D

In rP23351#40663, @elexis wrote:

(Can't really say whether artwork is unused or used if you account for the fact that a new scenario map using it could appear in a mod or repository every day if the quality of the artwork is fine.
Then it only needs to be decided whether it fits to the 0ad frame or not, and if so, it should be sorted logically I suppose. "old" should then be emptied by moving or deleting?

Yeah, I'm okay with keeping quality artwork for scenarios. Keeping unused textures and audio, dunno, cause it just makes the download size bigger, and people won't find them in releases without doing some thorough digging. The proof is, I'm still finding stuff in the repository, sometimes, that I made.

For example when I create new maps, I browse through the entire list of actors and filter out what I see might fit to the map and prefer unused actors, because that gives the map uniqueness.
That doesn't warrant keeping bad quality art however. I didn't look at this specific actor(s), so Im not sure if my 2 cents are actionable.)

If the actors are good why not. Sometimes there is no actor, just files.