Automation
Notifications you do not have to mute
The bar is that a reviewer can rely on them without also watching a chat channel, and that nobody has to turn them off to get work done.
Every notification carries its reason
Being asked to review something is a request addressed to you. Watching a repository is a standing interest. Telling a requested reviewer they are receiving a message "because you are watching" is how people learn to ignore notifications.
When several reasons apply, the strongest wins, and the reason is shown so it can be switched off on its own rather than all at once.
Nobody hears about their own action
This single rule decides more than any other whether notifications feel like noise.
A burst is one message
Ten comments on one pull request in five minutes is one email. The window is per person and per subject, so activity on an unrelated issue is still its own message, and a conversation that runs all afternoon does not arrive as one enormous digest at the end of the day.
Subscription is implicit, and unsubscribing is absolute
Opening, commenting on, or being assigned to something subscribes you. Unsubscribing from a thread outranks every reason, so a later mention does not quietly bring you back.
Notifications people keep switched on
Routing rules are pure functions, tested against the combinations that produce noise.