There is a lot more Necromunda activity over on Yakromunda (not sure how much, been too committed to other things to get back around to Necro stuff), and they have a fan-made Necromunda Redux, which is massively better than Underhive. Not that it was difficult.
I personally however stick to the Old Hardback. I typically only add post-underhive things such as scenarios, Can You Hear That? (Nik Jakos' masterpiece) and the High Impact rule.
But the Necromunda fan version is rather good. Activity is there mostly because Yakromunda has a few things over us. I'd still like this forum to do well, and that's probably why Lex and the other guys are trying to have this forum to provide more support.
In the mean time I do have a thread pinned in this section with links to most of the Specialist Games publications from before and after Underhive. As far as I'm aware, the stuff still works. Yakromunda also has some stuff stored on it.
GW have left Specialist Games alone, but IMO that's the best thing that could have happened us. Just look at what the likes of Matt Ward has done to the cores... yeah, I'm overjoyed the worst we got was Underhive. Now the people who know what they're doing (i.e. the fans) have control over SG. As they should have.