Is there a master list of plug-ins or extras included with each version of Sonar?

A number of plugins that come with Sonar are simple VST dll-type, and you can copy them off to some folder and dump them back into wherever you want them to reside - adding that path through the Plug-in Manager to get them to show up again.
 
Some of the older DX-type plugins may not work any more in either later versions of Windows, or in a 64-bit Sonar environment.  Even some 32-bit VST plugins may not work in a stable manner (or at all) if they are used with either BitBridge (Cakewalk software included with Sonar that lets 32-bit VST plugins run in 64-bit Sonar) or with JBridge (3rd-party software that lets 32-bit VST plugins run in 64-bit Sonar).
 
Another consideration is whether or not those earlier versions of plugins are 32-bit, and whether or not there are either later updates to them in later releases of Sonar, or perhaps there might be 64-bit versions of them available should you choose to run 64-bit Sonar now.
 
And, there are plugins like Dimension Pro and such, that have their own installation process. Those generally do not uninstall if you remove a Sonar version, as they would have their own uninstall process.
 
Hopefully some other folks will jump in here with some better responses for you.  I myself did not keep anything prior to Sonar 8.5.3, and once I became happy with X2a, I even removed 8.5.3 as well as X1d.  I also try to run with only 64-bit plugins these days whenever possible, but a DO have a 32-bit install of X2a present if I have any issues with including the remaining 32-bit plugins in my 64-bit Sonar X2a environment.
 
I have been copying and pasting ARP2600VA and MinimogueVA for years now, both of which are freeware VST synths that I happen to really enjoy, and I just have them saved off in a backup folder and plop them into a VST Plugins folder whenever I move to a new computer, and they cheerfully run just fine that way.
 
I don't know if any of the above is of any help whatsoever, but maybe it will help you to think of some things you might not have, and that you come up with a workable game plan for cleaning things up.
 
And, as long as you have the various installation files for prior versions of Sonar, you can always recover any particular plugins by doing a custom install and selecting just the plugins you wish to recover, while skipping the rest of the components.
 
Bob Bone