Niven conceived the "ringworld" idea as a practical compromise solution to the "Dyson Sphere problem": you can't actually live on the inside surface of a Dyson Sphere, as there is no way of giving the "floor" gravity; natural gravity inside a hollow sphere is nonexistent (no matter how thick and heavy you make the walls, the gravitational pull of the stuff beneath your feet is precisely cancelled out by the pull of stuff on the opposite side of the sphere, hanging over your head) and spinning a Dyson Sphere for pseudo-gravity would simply pull all the air and liquid towards the equator, resulting in a million-kilometer-deep equatorial ocean, two thin strips of habitable land around the shoreline, and two vast airless polar deserts. So, Niven argued, why not just skip the whole Sphere thing, and just build those "two thin strips"? Then you could spin it for gravity, and still have gazillions of square kilometers of lebensraum.
Shortly after Ringworld was published, astrophysicists pointed out a critical flaw in its design: just like a Dyson Sphere, a Ringworld is not gravitationally bound to its star. Or rather, while the ring is held in place axially by gravity so the star can't simply just drift away, there's nothing to prevent the star moving laterally (off-centre) within the plane of the ring; eventually, after a million years or so, the star would drift too close to the ring wall and disintegrate it. Niven's concept of an inner partial-ring to create a day-night cycle would only hasten the system's demise, as the star wouldn't have to drift off-centre for as far before the inner ring was destroyed, and when that happened, bits of the inner ring would go flying out and smash the outer habitable ring to pieces. A ring (or set of rings) would need to be regularly firing engines, to keep the star in the centre. Niven's sequel novels explore this need for constant attitude adjustment, but it basically means that an uninhabited, abandoned Ringworld would not survive long-term.
Iain M Banks' Culture novels explore a smaller, more practical form of ringworld, the Orbital, where the ring doesn't have a star in its centre but orbits a star just like a planet; the ring is tilted with respect to the orbital plane, to let sunlight in, and the ring rotates once a day to give both gravity and the day-night cycle. At 3 million kilometres diameter, a Culture Orbital is still an impressive megastructure, and would have a much greater chance of surviving for millions of years after the civilization that built it vanished.
The question of "have they secretly added such things to the game", I suspect the answer is "probably not". A Ringworld would require significant work from the game designers, from graphics to sound to Stellar Forge programming. Making them procedurally-generate would mean that FD would have no control over how many and where such things would pop up and would not themselves know where such things would be located until and unless players found them. Making just one or two, and manually hiding them somewhere in the 400 billion stars, would potentially waste all that effort put into designing it if no player ever discovers one and sees what they have made. So if they ever did make such things, put them in the game, and no-one found them, FD would - eventually - start dropping clues to their existence.
