The only forum I believe is going is the Optimatch forum due to it being useless since Bungie has no control over any aspect of halo. The Universe forum is definitely not going anywhere due to it being the forum where discussion on all of Bungies game universe goes. Bungie created halo. If we couldn't talk about non bungie halo related stuff then they've been pretty lax as cryptum is a 343 issued product and they have no qualms about that.
I doubt forums like Reach and ODST will go since Bungie created those games. 343 IMO aren't the type to tell them to shut those forums down.