Site policies are explained more at Policies. This page contains policy issues for how to manage pages.
In general, pages should only be deleted if they cover topics that are not of any relevance to VisIt users and no longer fulfill their original mission. An example of this pages that describe source code commit practices from old source code management systems that are not in use today (Clearcase, svn).
Rather than deleting a page, sometimes it makes sense to keep it for historical purposes. An example would be pages that deal with older hardware platforms that contain useful techniques for building the code that could be applied to other platforms. In such scenarios, mark the page as "Historical" by placing a tag at the top of the page: