Have they said it won't be on Windows 7? I would expect them to release it on Win7 to try and keep their market share. I hope they do as I am building a WebGL application for work :)
So far it's at least safe to assume MS support current and previous releases. As this is a feature pack to w8 and not w9 I should be ok so assume ie11 will see w7.
But who knows, MS seem to think ie is some carrot to lure people to them. It could as such be crippled to w8.