It's best practice to use either the Steam Workshop, Pakrat, or VIBE, all of which do work.
I personally use Pakrat, but some issues arise where the file can become corrupted and takes several tries to work.
I believe Valve has disabled the method of downloading individually because some default maps didn't come with one or have one. Can't find the proof of that, but I remember reading it somewhere, I'll see if I can find it.
But again, it does work but it does take a few tries for it to work properly with Pakrat, VIBE may work better.
Link to Pakrat
Link to VIBE