The answer is Yes, with the caveat that you need to be using "the right" player.
You can certainly watch 4K UHD VP9 content from YouTube on one, though I've not tried a 4K BR.
Thank you for giving heads up on video core next. I somehow managed to miss all news about it. Apparently we are right now on VCN 3.0, and it supports AV1 too.Both gen of RDNA have the hardware, AMD calls it Video Core Next. Drivers were, as usual, the issue. They added support for Netflix 4k back in 2019. As you pointed out, they had to meet compliance with Microsoft PlayReady 3.0 for DRM. Whether or not it gets nerfed in a particular driver release is anyone's guess.