Hello, I looked at WebKit code and they have a class to decode VP9 with VideoToolbox, which I used as reference for testing, but the code doesn't work on iOS. Maybe they are only preparing the code for a future update... Even on iOS 14.2 and the latest Xcode Beta, VTIsHardwareDecodeSupported(kCMVideoCodecTypeVP9) is returning false and although CMVideoFormatDescriptionCreate() returns successfully with kCMVideoCodecTypeVP9, VTDecompressionSessionCreate() returns error -12906 which means "could not find decoder". So it looks that either Apple is hiding something or YouTube is using its own software decoder.

Same here, email sent. Thank you! Pablo.