Limits of custom notification levels for reducing email noise in GitLab

Goal is to receive high-signal alerts such as failed pipelines and direct mentions while keeping overall email volume low.

GitLab supports hierarchical notification levels at Global, Group and Project scope, with more specific levels overriding broader ones. The Custom level allows selection of individual events like failed pipeline, fixed pipeline, merge request events, new note and your activity. Mentions and direct participation are known to trigger notifications independently of watch level in many cases.

Group-level settings inherit to projects unless a project overrides them. An unresolved behavior is whether built-in digest, batching or severity-based routing exists for email notifications. Without such features, volume from busy projects must be managed externally.

Does GitLab provide built-in digest or batching for email notifications? Does mention notification bypass Custom event selection? How does Group-level inheritance interact with per-project Custom selections for users who want quiet globally but alerts on critical projects?