What are the benefits you are hoping for?
I understand that pages in the proposed namespace will not show up in search results any more, unless I specifically search in this namespace. While this can be beneficial, it can also have negative effects. If old pages are excluded users will often get seemingly incomplete or even empty results, thinking there is nothing to be found, or the internal search broken. Personally I rarely had issues with outdated pages polluting my search results. More the contrary. If old pages show up that's probably because I was searching for exactly that information.
I hope we are not going to hide this namespace from external search engines, as this would make this content largely inaccessible.
Another issue I see are broken links between pages. MediaWiki relies on one mechanism to make sure links between moved pages still work: redirects. These would need to stay behind. This can become a problem when people start deleting the redirects, or using them for other content.
Personally I'm really not sure if a namespace is the best possible solution. mediawiki.org is more a continuum of content we collected over the years. Pages are rarely up to date. It would probably be less cumbersome to consider the existing main namespace an "archive" and only mark pages that are known to be up to date.
Kind regards Thiemo