We actually started putting something like this together a while back, just so we didn't lose the content: Deprecated Pages | Ignition User Manual
Admittedly, the link to those pages is somewhat hidden (all the way at the bottom of the sidebar in the user manual, next to the links to other user manual versions). Also, there's a lot of clean up that needs to happen for that section, such as listing when things were deprecated, and linking to the replacement features.