P.S., IMO the first-aid icon in the lower right corner of the compose window is confusing. I just clicked it and I still think it does something else. 😃
It is currently possible to disable all user email notifications, but it would be really useful to separate a digest email from other emails. Quite often forums are using paid services, such as mailgun, which have strict email sending limits, so it is quite easy to go over these limits when enabling email notifications (because all users want to subscribe to many topics). But digest is only sent daily, so it helps to avoid this issue because we will know exactly that every user will not receive more than 30 notification emails per month. But currently I don't see how it is possible to enable only emails for a digest, and disable it for other types of non-system notifications.
@arutemu Opening an issue on Github is the best way for us to track feature requests.
Thank you, I created a new issue on GitHub for this feature request https://github.com/NodeBB/NodeBB/issues/5644