@Kovid - thx for that, I will test your changes shortly.
They will handle the situation of a plugin like "Plugin Updater" or "Find Duplicates" becoming part of Calibre nicely, and in that situation it will be appropriate to remove the plugin from the forum plugin index page completely so it will not appear in the plugin updater dialog.
However there is still the other scenario of where a plugin gets renamed or combined into a new plugin. The most recent example is "Kindle Collections" which was previously known as "Create Kindle Collections". I will shortly have an additional scenario of "Goodreads Metadata" and "Goodreads Covers" being replaced by a new metadata source plugin called "Goodreads".
I have made the changes to plugin updater to support a "Deprecated" value on the index page, and placed an additional greyed out section on the
plugin index page at the bottom where such plugins will go to "die".
With the changes I have made, if it sees that you have one of these plugins installed, it gets a special icon and appearance as per the attached screenshot. It will show up on the "All", "Update available" and "Installed" views. If you either (a) install it's replacement using Plugin Updater, or (b) uninstall it, then it disappears and will never be visible again on the plugin updater dialog (well unless you reinstalled it manually outside of plugin updater).
I have also added support for multiple uninstall plugin targets, so my Goodreads plugin will have "; Uninstall: Goodreads Metadata,Goodreads Covers" on the index page and both those plugins will get uninstalled. [Note that I will only put that attribute on there when Calibre 0.8 is officially released, as it is valid to have all plugins side by side for users who are just temporarily testing both].
Before I push the new Plugin Updater version, any objections to the above?