The documentation site has been updated to include new (8.2) releases and to make links like these more stable.
Try this one: http://techcommunity.softwareag.com/ecosystem/documentation/webmethods/wmsuites/wmsuite8_ga/relnotes/8-0_webMethods_Release_Notes.html
And, if it isn’t there, just do as I do when I get a dead link - start from the home page and search or poke around.
At present, from the home page, you would look for webMethods > webMethods Product Suite > webMethods Product Suite 8.0 and 8.1 > Documentation by Product > ESB and Integration Server, and Process Engine > webMethods Product Suite Release Notes
#webMethods#webMethods-Suite#webMethods-Archive