Steam APIs have not significantly changed in the last 15 or so years. They're not exactly well documented, but they are relatively well understood because they haven't changed.
I'd say what they did right is simply not break existing functionality.
Exactly this. It feels like Twitch changes things for the sake of changing them. There is not reason why their changes couldn't have been backward compatible. It's just unnecessary churn for developers, who often are working for free making tools for your website.