What does "Ignore messages like this" do? How do I see what I've ignored? When I tap and hold on a notification in the notifications list, I get an option to "Ignore messages like this." I can't find any explanation, though, of what this means. Does it really mean, ignore all notifications from this app? Or does it mean, only ignore messages like this one from this app? (i.e. can one app have multiple "channels" or types of notifications?)

And the other side of this - where can I see what messages I have chosen to ignore, and possibly undo it, if I once again want certain messages?

UPDATE: In CM settings there's a "Notification Filter" screen that lists all notifications that I've chosen to ignore. (Settings > Privacy > Filter Notifications in CM11.) I still don't know what the filter does, though - it hasn't seemed to keep me from seeing any notifications.

(I suspect that this feature only blocks you from seeing notifications whose source and content exactly matches a past notification, but I can't confirm that so I don't want to post it as an answer. Can anyone confirm or deny?)
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.