Information is limitted as the contracts used for developers aren't shared, but the general understanding is that this only applies to Steam keys.

The one exception is the wolfire games lawsuit, which includes one alleged instance of Valve asking a developer not to distribute the game for free on their Discord when it is a paid product on Steam. Given the lack of detail, the single anecdote for evidence, the existence of other games where they are priced lower or free off Steam (I.E. Dwarf Fortress), its certainly not a widespread problem, almost certainly not in contract, if it did happen exactly as the anecdote suggests, may have been a misstep on the part of one employee, and may not have happened at all.

Of course, if Valve does do this, nonetheless mandated it, its an issue, but given that no one else has challenged them on what would be such a blatent anti-trust case, esspecially given how everyone else in the industry has been trying to take Valve's place for years, I think its unlikely.