Hello,
Most died or depopulated enough to be dropped off the maps as inconsequential. They may now be completely dead rocks, or homes to populations so small that they're effectively inbred or transient, or pirate waystations. The Star League era saw a lot of mega-engineering and colonization efforts on the fringes that basically collapsed as soon as the League itself did, because there were no longer any parties with the resources or interest in saving the most outlying worlds. Underpopulated and partially tamed, they backslid quickly.
Hopefully that helps.
Thank you,
- Herbert Beas
BattleTech
Catalyst Game Labs