Is it possible to publish a game on Steam as pay-what-you-want? Something in the likes of the Humble Bundle. If it's not natively supported by Steam, would it be fine (as in, not violating any contracts or agreements) to have the game published as free (or at a very low cost) and include a link to a page where you, if you so wish, could donate a sum of money?
Steam does not have direct support for this kind of model.
It's common to fake it on other store fronts (such as on iOS) by making the game available for free and selling some kind of renewable microtransaction that is effectively a "tip jar."
Steam does not mandate exclusivity from you, and many developers also sell their game elsewhere. There are plenty of examples of developers who are putting games on Steam and also receiving donations outside of Steam, for example from Patreon or the like. There does not appear to be anything in Steam's user agreements that contradicts such an approach.
Another work around is selling empty, support the devs DLC packages through Steam.
Inspired by our community constantly asking us for ways to pay us for the DLC’s we’ve also come up with Coffee for Coding
Note: I have not read the fine print of Steam's developer or user agreements, I'm inferring permissibility based on prior examples.