9.0c games ought to still work. The only issue is that any features that are only provided by 9.0c won't work. Games should be able to fall back to an older feature-set. The main reason the new version of DX is still required is just because of what code the game expects to find, what files, what API hooks, etc., which changes between versions. They still provide the same features as DX9a or b, as well as c, but how they're accessed changes a bit. Since they want to write for the latest version they can, without also having to write it to be able to handle all the older versions, they just require the new version.