Is your history off or was I always misguided? I thought Steam effectively launched such HL2 which included the release of CSS. CSS is what required it rather than 1.6, no?
CS 1.6 definitely required it. 1.5 was the last version of CS on WON. It was the flagship game to have Steam for until Half-Life 2. And yes, CS:S also required it.
CS 1.6 required Steam and was released together with Steam 1.0. You could say that it was a soft launch to get Steam ready for HL2 one year later.