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.

Thanks for the hint, I think it is working now.

Thanks for that!
(I noticed this for a niche reason: I use the bookmarks system to make a note of mods I want to install when I'm back home, but I don't have Space Age yet, so my list is currently filled with dozens of Space Age mods and I hope to filter out the ones that are Space Age only when looking for ones to install.)