Though Vulkan has video decoding built in, so eventually everything will hopefully unify to use that and we can avoid needing multiple separate subsystems depending on vendor...
It looks like [0] there currently exist extensions for H.264 and H.265. I don't know anything about Khronos politics, but I expect there's nothing that would stop e.g. Google from proposing similar extensions for VP9, AV1, etc., besides the need to actually get it implemented. (The H.264 and H.265 extensions have only AMD, Intel, and NVIDIA employees listed as Contributors.)
