At this point, in 2021, netflix and YouTube both support speed control on web, android and ios.* this is clearly a popular feature. Will roku be implementing it soon?
*if you are one of those people who feels compelled to comment that nobody wants speed controls, please read my first sentence again, then go away.
You're asking about app functionality, right? That would be within the app. Roku doesn't build the apps, just like Apple doesn't build the iOS apps and Google doesn't build the Android apps.
DBDukesRoku Community Streaming ExpertNote: I am not a Roku employee.
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YT speed control functionality is enabled on all 4K/HDR-capable player models (w/different versions/builds of the YT app), though there are reports of individual units not being enabled yet.