Shemeska
Adventurer
I thought it was 3.x that really did the hatchet job on the factions. Granted, the old 2e versions had grown a bit stodgy, but I thought the 3e version just chopped stuff out without really adding anything. 4e then added nothing again, but just grudgingly gave minimal shape to a scant few favoured factions, so neither 3e nor 4e are due any honour for it, IMO.
3e pretty much left them out initially, but mentioned that factions existed within Sigil. In 3.5 several of the factions were detailed and given Prestige Classes in the Planar Handbook (Sigil was also given a decent post-Faction War writeup in the same book).
4e (largely as a result of Rob Schwalb's really good writing) had some material on some of the factions in Dragon/Dungeon, but at times the presentations of material from Planescape (Brian James added some material back in with his work in 4e's Demonomicon, and other bits popped up in other books) suffered to an extent due to being forced to alter itself to fit into the 4e World Axis cosmology where half of the alignments had vanished along with many of the planar races (or been turned into things different in all but name).