Pre-Space Flight worlds

[Phantom000]

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?

[AlphaMirage]

There are several worlds which are expanded on in Interstellar Explorations and the Explorer Corps books. You are correct however that most of them are very deep in the periphery. Additionally there are some worlds within the Chainelaine Islands and Barrens (former RWR) that have lost their tech. I imagine that out of the 'lost' worlds post First and Second Succession War probably 50% of them might have devolved and thus been removed from maps by COMSTAR.

[RunandFindOut]

Also remember that the maps and lists we do have are CANONICALLY not exhaustive.  They're noted to only include worlds important enough to have an HPG on them, or worlds that despite not having an HPG are still important enough to put on the map (not many of those on the map).  So given that even in-universe they don't know how many colonization attempts were made over a period of several hundred years by all sorts of different groups, and that it is a known thing many groups went out to colonize and either were never heard from again or were lost contact with after a while.  You have all sorts of possibilities for lost colonies even independent of the known "lost worlds" as those were places important enough to get on the map at one point only to be taken off for one reason of another.

So it's very possible to run across a primitive world even inside the inner sphere.  All it takes is one major incident of the wrong sort and a colony can find itself still able to provide for itself but no longer able to produce advanced technology.  The inner sphere moves on, they get put on a list of failed colony fleets that never reported back, and then hundreds of years later a Jumpship finds themselves making contact with "the colony that time forgot."  There are probably trashy holovision shows about it in the Sphere, right alongside the "invasion of the space Kaiju" and "Star League remnants beyond the periphery secretly protecting us from the evil aliens" stuff.

[Frabby]

And the House Arano sourcebook has since pushed the envelope of what is regarded as "not notable". We can now say with confidence that truly devastated worlds which have been reduced to pre-spaceflight tech levels are omitted from maps by default. Only few of them are on maps; conversely, some are hidden on purpose (e.g. Alfirk, Haddings).

We do know they exist in not-insignificant numbers.