Not on games per se but almost on mods. Yes, there are other platforms, but the majory and newest are on Workshop. And they make it hard to download them, if you don't own the game on Steam (and Valve has it in their terms that they own the mods hosted on Workshop).
So one could argue that they use the indirect peer-pressure approach to market dominance, similiar to Google on Android.
I wouldn't complain if they just had a "Download" button on their web version. But they don't, you have to use the finicky steamcmd intended for server administration.