Thanks for documenting this here, Tyler.
This information doesn't appear in the documentation where I would expect: https://support.atlassian.com/bitbucket-cloud/docs/manage-email-notifications-for-watched-objects/
The page about watched object notifications doesn't mention pipelines at all.
I think it's important to mention them explicitly because a common pattern on a small projects is to have a single user committing directly to the master branch with a single pipeline. That user won't receive any notifications about their own commits, and there are no PRs, forks, issues or wikis to notify about.
But there are a lot of pipeline failure notifications. So, so many failure notifications... 
To my understanding, this is a violation of the EU's GDPR (General Data Protection Regulation) that has to be addressed.