Thanks to the involvement of Richard Siddall, Anne Ogborn and Fastly the CDN for www.swi-prolog.org now redirects to HTTPS. This should protect your privacy and ensure downloads cannot be hijacked by man-in-the-middle attacks.
The url https://swi-prolog.org (https, but without www) does not yet work. The simple http://swi-prolog.org does work.