Has anyone used just paper to make terrain pieces for their wargame, sort of like pop up sets like pic related?

I have, a long long time ago during my youth when 3D printers were science fiction and I had even less money than I do now. I remeber making foldable terrain for easy storage. Worked pretty good. The easiest is to make box terrain tho.

>>623346 (OP)
Yes.

Paper figurines are also a thing.

>>623346 (OP)
Yes, there are a handful of people publishing war game and rpg papercraft. It's a shame it's not more popular as it has some advantages over 3d printing.
https://preview.drivethrurpg.com/en/publisher/6291/paper-realms
https://preview.drivethrurpg.com/en/publisher/346/fat-dragon-games

>>623346 (OP)
You'll be wanting this stuff:
https://www.stonehavenmini.com/collections/pop-up-terrain

Yes but DND maps

There's plenty if you know where to look. Especially on DriveThruRPG, where terrain is often free or "pay what you want".
This one is one of my favorites.
https://www.drivethrurpg.com/en/product/78956/hovel

>>627690
This one has removable roof as well. It's a bit tricky to put together, but well worth it.

These are from this file, printed on colored paper so it's more vibrant.

https://mega.nz/file/72pnEASb#ASWrF7cSdJ74xI1OPtRVy-2C9fxsEQHTNHjskUCWNKY

These are a bunch of David Graham models, some are free, some are simply cheap. They sometimes come with alternate roofs, like the thatched cottage also coming with a red tiled roof file. And there's often those boxes as filler, which I appreciate.