Mailing list archives, release announcements and Stack Overflow answers link to the site for years. When you move, rename or remove a page, redirect its old URL to where the content is now.
Add a line to .htaccess at the root of the repository:
Redirect 301 /start/download.html /versions/
The first path is the old URL, the second where it goes. A fragment in the old link, like #maven, is kept, so give the section on the new page the same id.
For a whole family of URLs, use a pattern:
RedirectMatch 301 ^/start/wicket-([0-9.]+)\.x\.html$ /versions/wicket-$1.x/
Jekyll copies .htaccess into content/, and the Apache web server of the ASF applies it. Your local preview and design previews on GitHub Pages do not, so test a redirect after it is published:
curl -sI https://wicket.apache.org/start/download.html | grep -i location