What does "Ignore messages like this" do? How do I see what I've ignored?
After some testing, this feature seems to hide future notifications when the same app shows a notification with exactly the same text again in the future. This means you can't filter messages like [filename] download completed from the download manager, [app name] installed from the app store, or Level up! You are now level [number] from Google Play Games, since they have changing content.

As such, its usefulness seems pretty limited. There's no good way to choose which notifications you get from most apps, unless the app has those options built-in.