To remember these great games and the memories millions of people have made in them!
What do you mean? There has been no announcement to say that the legacy forums are being removed.
I would guess that 343 Industries will keep this sub forum. However, the most likely thing that may happen in the future is that few more game specific forum sections would be merged into this forum section.
Previously, most of the games in this forum section used to have its own game specific forum section like Halo Reach and Halo 4.
Campaign mod will remain playable so this forum need to remain open.