This page redirects to an external site: https://wordpress.org/documentation/wordpress-version/version-2-9-1/
WordPress Version 2.9.1 was comprised of several bug fixes including scheduled posts and pingbacks not being processed correctly due to incompatibilities with some hosts.
See the list of changes for 2.9.1