The shift appears to have taken place on Dec. 6, according to a Reddit thread delving into the issue. Google has been pushing the open and royalty-free VP9 codec as an alternative to the paid H.265 spec since 2014, but has never said that it would stop offering 4K video on the YouTube site in other formats, like the Apple-preferred H.264. Videos uploaded to the service prior to Dec. 6 in 4K resolution can still play back in full 4K resolution on Safari from the YouTube homepage. Additionally, Mac users utilizing Chrome still have the ability to play back new videos in 4K, as Safari is the only holdout among the major browsers that doesn’t support the codec.
Apple should adopt Google's royalty-free VP9 codec in Safari. Not because it will benefit Apple or because it will allow for 4k streaming on YouTube, but because it is the right thing to do for the preservation of a free and open web.
I understand Google is a competitor of Apple, and there is currently no VP9 optimization in in Apple's hardware. But time channges all battle lines, and hardware can be improved. Keeping the web free and open is a choice we need to make now if we want future generations to enjoy the same freedomes.