Some were added years ago, but are no longer stable enough for current versions of Gramps. Or are still in development. And are therefore excluded from the Addon list (by using “include_in_listing = False”).
If nobody is working on them, it might be an idea to put them in a ‘development archive’. Then they can be reused later. Otherwise, translators should keep checking them periodically for changes…
HtmlView was used by the old geography views. Instead of removing it completely we moved it into the third-party addons repository just in case anyone wanted to continue using it.
MongoDB is an experimental database backend. The code may be of interest, but it won’t see any further development until the raw database format changes to JSON.
For the new Addons Manager I am planning to enhance the plugin status and add intended audience and maintainer details. This should make it clear if an addon is no longer maintained or is not intended for general use.