When a plugin is removed from the community repo, the user isn’t notified. Instead they’re allowed to continue unknowingly using an unmaintained plugin.
For example, I only found out that Customizable Sidebar had been merged into Commander when I opened its description page to get the link for someone and the page was missing with no explanation. I had to search the web to figure out what had happened, and it was an annoying hassle.
More recently, someone in Discord mentioned that a plugin I have installed (but turned off a while back because of bugs) had been deprecated.
These aren’t how I should find out — Obsidian should tell me.
Minimally: when checking for updates, also check for installed plugins that have been removed from the community repository. Notify if any are found.
Ideally: Provide a way for developers who remove their plugins to leave a message.
Current workaround (optional)
I just find out by happenstance, but I suppose you could manually check the description page of every installed plugin every so often to see if it still exists.