Though there are mods, they aren't accurate to the original content.
You can't 'turn off' the rebellion expansion and play the original Sins as it was initially released before coming to steam.
Basically you can still get around it by having other players you are playing with download the custom map you made off a google drive and manually put it in the right folder - however this doesn't help the already barely existing multiplayer scene. Multiplayer map sharing is non existent. Yeah we get that it's an old engine, but it's still a sore point of the series for me. This fortunately occurs only in large battles, and can be counteracted with in-game options limiting fleet sizes, but it's infuriating having your 4 year old GPU and CPU sitting at 10% of their power, while the game creeps along at under 20 fps in a big fight. The game engine just refuses to use the full extent of any hardware you throw at it, opting to instead run at a low framerate. This is offset to an extent with mods and very straight forward and easy to use map editor allowing you to create your own maps to play by yourself or with friends - but the unfortunate reality is that without a campaign or a story, this game misses out a massive chunk of potential and draw for new players. Without spending paragraphs raving on how good it is, basically think of an old school RTS from the 'golden era of gaming' and you have sins.
