I really think Valve have become the de-facto owners of the “don’t be evil” motto nowadays, even if they don’t advertise themselves as such.
They got and have maintained that monopoly (I'll let others debate the merits of that wording) by being very very good to their users, which doesn't make the existence of the monopoly evidence that they aren't saints. If they were maintaining it through anticompetitive means, sure, but I've never seen anyone claim that they are, even Epic (who would definitely be making noise if they thought they could get anyone to listen).
The desktop video gaming ecosystem is in perhaps the best shape possible: there's one clear winner at the moment who makes all customers very happy, with a few runners up hedging against that winner becoming abusive after all. If Steam became worse than Epic it wouldn't take long for Epic to overtake them, but as long as it's not worse it's nice that everyone has agreed on a standard platform.
They did get sued for having "anticompetitive restraints on pricing" and "Federal Judge John C. Coughenour ruled that those claims were credible and that Steam gamers can claim compensation for Valve's illegal monopoly, but gamers, unlike developers, must file individual arbitrations to do so."
So, yes, it's been claimed and legally found that they have at least some anticompetitive practices, at least in the USA.
(Quoted text is from https://www.bucherlawfirm.com/steam-case-explained)
It's also possible that some gamers did actually get money from Valve via arbitration, so they could've been found to have acted in an anticompetitive way, separately from the lawsuit. I've not been able to find anyone specifically saying that they did the arbitration, though.