While it is still a browser policy decision the WebGL WG, like the CSS WG, could come up with recommendations for shipping/making available non-final things.
Unlike the CSS WG, I don't suggest getting too detailed. The question really comes down to "is it more harmful to release something that can possibly change behaviour in the future without a way to distinguish new vs old content, or to pollute the namespace with prefixed crap that might force other browsers to implement it?"
Right now the wind is blowing in the direction of the former. However, I have a feeling as soon as something awesome gets adopted by one engine then gets changed after shipping, the wind will start to swirl. There isn't a great solution :(
I'd be fine with a recommendation of no prefixes for WebGL. We've sort-of informally agreed to that when exposing the rendering context for WebGL2. We could do the same for extensions, but I'm already on the record as saying we should really avoid extensions as much as possible.
And as Ken pointed out, I removed the prefix for this extension in WebKit.