In the 1970s Gerard K. O’Neill wrote and published his seminal work, The High Frontier. In it O’Neill first paints a picture of habitats floating in space, with people living on the insides of cylinders kilometres long, and then describes how one could accomplish this from an engineering perspective with the Space Shuttle, then still in the planning stages.
The Shuttle turned out to be more expensive and less reusable than it was optimistically lauded to be, but it stands to reason: where are the space settlements? Certainly not in low earth orbit.
But they don’t have to be. What once pushed the envelope of the technically feasible is now well established as doable. The Mir was an initially Soviet, later Russian space station from 1986–1996, crewed for almost ten years. The International Space Station took 12 years to construct and has been crewed since November 2000. By the time it too will be decomissioned almost a whole generation will have passed since the first crew boarded it.
Will humans have sustainable settlements off Earth by 2100?
This question will resolve positively if there is a settlement(s) off-Earth with over 2,000 5+ year residents that are self-sufficient in production of energy and foodstuffs.