From Wikipedia regarding VP9[1]: "Parts of the format are covered by patents held by Google. The company grants free usage of its own related patents based on reciprocity, i.e. as long as the user does not engage in patent litigations."
Of course Apple refuses to add VP9; Google demands that Apple would never sue them over any patent. Any. It looks like Google/YouTube caved in, and has/will start encoding (the most recently uploaded) videos in AV1 codec, which will work on tvOS. Hopefully that will translate to Safari on macOS and iOS for those who use it.
