But wait. What if you put something not in the demo and then load it?

Yes, you can. I've tried it before and I managed to get Steam exclusive features on shapez.io.

Yes. I'm almost to freeplay and decided to export my game for the challenge. Went to the website and loaded up the savegame. Here's a pic.

I think I remember having a save on the demo before wires update came out. then recently I tried importing a save to the demo