In the mod portal search page (on the website and in-game) there is an option to filter out Space Age only mods from the search results (it's the "🚫" symbol next to the words "Space Age" at the top of the left part of the search page). However, despite this mod only being usable with Space Age, it must be missing a setting somewhere because it's still showing up even when I filter out Space Age only mods.

I don't know how to mark a mod as such, but an example of a mod that is filtered out as I'd expect is the "Planet Pelagos" mod.

From what I remember, that flag is pretty finnicky to deal with (for adding and removing from mods on the mod portal), and I remember seeing plenty of Space Age mods that don't have the flag, even though they should.

As much as I wish that flag on the mod portal would just automatically apply if Space Age is a required mod (which makes sense to me, I don't see why it should be any other way), this is such a minor issue that I probably won't bother fixing this.

Worst case is that I accidentally apply the flag to a mod that doesn't need Space Age (because I tend to copy my old info.json when creating new mods) and it gets filtered out by the tag, limiting its audience and thus nobody uses it; while the opposite side of the problem is the current situation, where my Space Age mods don't get tagged as such, and... that's about it.

There's not really a downside to having non-Space Age players see a Space Age mod (especially when they are pretty obviously Space Age, people aren't going to accidentally download a planet or space-adjacent mod, thinking it's somehow works with base game)

It's a pity it's so tricky to do. Thanks for checking anyway.