In Battletech they talk about worlds being 'bombed back to the stone age' especially during the succession wars, which is why some worlds don't have their own battlemechs or dropships and the only examples of advanced tech are strictly off-world. But what about a planet that, for some reason, was reduced to pre-space flight, maybe even pre-industrial levels and lost contact with the rest of humanity?
It seems more likely to happen in the periphery, especially in the deep periphery maybe somewhere outside clan space, but it might happen in the Inner Sphere. I could see a few star systems that just happen to be in an unimportant region away from most of the important trade routes.
Has that ever happened?