The data at CanIUse.com seems to suggest they're going backward and used to have an installable plugin:

https://caniuse.com/av1

see also:

https://www.thurrott.com/forums/microsoft/windows/thread/you...

Which seems to be claiming the software fallback has been suddenly yanked, temporarily breaking YouTube which fixed it by serving VP9 instead but maybe AV1 hardware decode is still working?