Valve allows developers to generate activation keys for their games and sell them on other platforms, where Valve gets a 0% cut. This is how you're able to buy games from places like the Humble Store and activate them on Steam. Their agreement does technically require that you don't sell at a lower price on other platforms, but as far as I know it's never been enforced.
There's a lawsuit ongoing about Valve threatening developers with delisting if they sell non-Steam copies of games (that's NOT Steam keys, but, say, a version on the Epic store) on other stores.
Can you provide a source for this? This is the first I've heard of anything like this and searching only gives results about the game delisting due to payment processor problems from a few months ago.
edit: after some additional search tweaking this is most likely in reference to Wolfire v. Valve, which is now a class-action suit.[0] The argument seems to be that Valve is engaging in anticompetitive behavior by disallowing developers from reselling Steam keys for their games for lower prices on other platforms, not selling the games themselves on other platforms. So this may or may not be what the parent post was referencing.
