I believe MacOS has full support. Windows now has support available as a free installable plugin.

Mozilla said that the current decoder state was unacceptable but if Google Research wrote them a Rust version that met their shippable criteria they'd ship it. That's planned to be done this summer. So I'm optimistic Firefox will ship support (courtesy of Google Research) within the year.

That would just leave Chrome support and Windows to ship by default. I'm hoping Firefox's support would cause pressure on the Chrome team.