They used to be 30%.
And Steam gives devs the option of selling Steam keys on their website without the cut, with the only rule that they can't sell it for less on their website than on Steam. So Valve only takes a cut oft their platform leads to a sale, users can still use the platform to play the games without Valve taking a cut.
Neither Apple nor Google allow this afaik, and I don't know enough about other platforms to know if this is common or unique to Valve.