Yeah, in theory it's available, but good luck actually getting your browser to use it to HW decode YouTube videos under linux out of the box.
It usually works fine with video players like mplayer but not in browsers.
There are endless threads online about this not working. I for one, never managed to get it to work in any browser. What a mess.
Chrome doesn't support VA-API, so do not bother trying.
Some distributions do have Chromium builds with VA-API support enabled, but YMMV. Chromium uses X11 only and libva requires dri2, so it doesn't work under XWayland, which supports only dri3. (=> Chromium with VA-API works only in native X11 session)
Firefox does support VA-API. There was a period of time, when it did only under Wayland, but nowadays both X11 and Wayland should work. When running under Wayland, use the native version and /not/ via XWayland, because the same thing as with Chromium applies.
On the Firefox side, Redhat and Suse people did the work to support video acceleration.
VP9 should be available since Mesa 18.1 (or so) for VCN-based GPUs (i.e. Raven Ridge and newer). VP9 is not supported in hardware on UVD-based GPUs (i.e. discrete Vega).
VCN 3.0-based GPUs (i.e. RDNA2, the 6000-series APUs) also support AV1 decoding.