Pretty Simple - If you open a session that is missing a plugin, one of the options (besides visit Avid Marketplace) should be to search in the "Plug-Ins (Unused)" Folder.
The reason is that now Plug-in devs are issuing upgrades with new unique names, rather than just updating the version. For instance, I have Clariphonic DSP, and Clariphonic DSP Mark II (newer version) in my Plugin folder. I will probably never instantiate the old version again, but I open an old session, I need the session to find the old plugins (this goes for Ozone 5,6,7, old versions of Kontakt, etc).
I would LOVE to remove these old plugins from my main Plug-Ins folder, since they are a drag on the system and my productivity when looking through menus (thanks for Plug-In search Avid, but it only goes partway). If Pro Tools was able to ask to search elsewhere for those old Plug-Ins if it needed them while opening an older session, this would be a great help.