Safari 18 has dropped support for vp9 decode (ie youtube 4k)

After upgrading safari recently, I found I've lost the ability to decode vp9 videos. I've also just upgraded to Sequoia and this has not restored that functionality, and the webkit preview doesnt do any differently.

Runnin MediaSource.isTypeSupported('video/webm; codecs="vp9"') returns false in safari, but true in chrome and firefox. I know I'm not on an old intel processor, but I've been able to handle youtube 4k fine for the last few years and I'd really like that back, and I'm confident I have enough juice to want to expend the extra energy decoding.

I've had a look through the webkit feature flags, nothing there seems to be disabled regarding vp9.