On the 3DSL page for this game, the link to the free demo on Steam instead leads to the store page on GOG for its version of the full game.
The free demo of the game is still available on Steam. You can easily add the demo to your Steam account without downloading the game by using the "app_license_request 907740" command in Steam's console or in the console in SteamCMD. "steam://install/907740" also works to add the game to your account and install it. You may want to add that information to the webpage for the game.