Well some maps are outdoors and use terrain. (not sure exactly what spatial partitioning they use but I'd assume an octree or something like that). BSP does stand for binary space partitioning, there is still a bit of BSP brush-based geometry used for levels (usually the shell is done in BSP and then certain parts replaced with meshes and details added with meshes).

You can't use BSP for terrain (well, technically you can but it's very inefficient). But there's nothing to stop you from having part of your level in an octree, part of your level as BSP, part as something else...

But the BSP tree stuff is probably mostly there for legacy reasons, there's better data structures to go with these days (that is a massive topic in itself).