They made a new portal in HTML5 and JavaScript. When you access the former one (that was in Silverlight), there's a link at the bottom inviting you to change.
IE9 was optimized for rich HTML5 and JavaScript applications and IE10 on Windows 8 won't even run Silverlight.
I should clarify that it's Internet Explorer _on Metro_ that won't run Silverlight (as far as is known). And, in consequence, ARM tablets won't either (because there will be no desktop mode, just Metro).
On the other hand, the new XAML-based API will be a somewhat easy upgrade path from Silverlight app to native app.
IE9 was optimized for rich HTML5 and JavaScript applications and IE10 on Windows 8 won't even run Silverlight.