Thank you for your feedback. I will forward your suggestions to the team.
You should know that even though the Developers Guide may say it hasn't been updated since Dec 2011, I can assure you that it has. It is constantly being updated and contains all the information for the latest release. There is also an RSS feed
you can subscribe to with the latest document changes.
There are no plans to 'version' the blogs or forums as the majority of them still apply to the latest version. For example there are several blog posts about extending widgets by customizing the widget template. These were written for version 4.x but are still perfectly valid for version 6.0. So filtering out something because it was written for an earlier version could make you miss the solution you need.
We try to keep the marketplace up to date as quickly as possible except for third party items which are beyond our control.
I hope this helps.
the Telerik team
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