Now user email changes will trigger the webhooks. @gpetrov yes, it will trigger only after the email is verified. And like all other webhooks, it won’t include the details about what part of the data is changed. It will trigger the user_updated webhook event with the user data. If you already have the previous values then you can figure it out by comparing.
https://github.com/discourse/discourse/commit/1460d7957c5d9b9300034e5e36675cf44cc3bc0f