Well other than multiple posts that shouldn't be there ( 5 times in a row @_@ )
Anyways, custom maps work in CSS Beta. Go ahead, try it. Highschool, Assault Redux, Jail Maps, Deathrun, Gun Game, Minigames, etc.
They all work. Why? Lets look at DoDS, a lot of custom maps were made fine before the OB update to DoDS. Those old pre-OB maps still work. The engine doesn't ignore the maps, it looks at the Engine Version Number in the VMF when it was compiled. With that code the engine can decipher how to read it and if anything old is on there. So entities that may have changed names, the engine will recognize that it was old and that its called something else in the new version.
In short custom maps work just fine. Plug-ins on the other hand, may be broken due to this update.