This used to be cyclic in legacy. (I can't speak as to memory of the last few worlds, as I do tend to use client mods.) I'm not sure what happened, but once you hit the top speed you're capable of, it doesn't go back down. Is there some key combo that was added I am simply not finding, or is it actually missing?
Beyond the missing key combo or cyclic toggling of move speeds, I like the method used in the mods--key for each speed so it's easy to switch from sprint to walk.