I see that email shouldn't be enabled by default (in fact neither web nor email are).

I don't really see a reason email should be disabled entirely though. If the user wants to opt into email notifications, why not? mention-failure in particular is still relevant information even if they see it a couple hours later, and they can go back and fix it.

If we want to, maybe only mention-success should be blocked, not sure.

See "Allowed notification methods" (can the user use this notification method if they choose) vs. "Enabled by default" at https://en.wikipedia.org/wiki/Special:DisplayNotificationsConfiguration .

The only other one like this is emailuser, since that's inherently redundant (an email telling you you got an email).