Being able to send email notifications from a Drupal site is great, but sometime the result can be a lot of noise in an administrator or editor's inbox. Message Digest allows for notifications to be sent on a fixed cadence (e.g. daily or weekly) compiling together all the notifications for the period. This module is part of the Message stack, and because it exposes no UI, if you want your users to be able to choose the frequency of their notifications, you'll need the companion Message Subscribe Email Frequency module (not available for D9 as of this writing).
Link