There’s been a lot of articles and discussion about it since this lawsuit first showed up, and the general gist that I’ve seen is that:

• It’s not codified in their agreements, but
• They can / have in the past either removed products from the Steam store entirely or removed them from curated marketing and promotion in response to the game being listed elsewhere at a lower price.

They seem to handle it on a case by case basis, but in those cases it’s definitely not been restricted only to the sale of Steam keys. They just don’t have any firm legalese to refer to here that I’m aware of.