If it's approved as a web standard, I would expect them to support it basically forever, since otherwise websites would stop working. I don't think there's any plan to deprecate the previous standard (WebGL) that came out around 10 years ago.
There's basically zero uncertainty around this. It would break the internet. As you probably know it's extremely difficult to make even tiny backwards-incompatible changes to features that have become web standards and have been implemented on all major browsers.