See the RFC.

The goal of this project is to build the foundations of a new notifications subsystem, and we're starting with system-generated notifications. These notifications will be triggered by coderd exclusively, and will cover scenarios like password resets, workspace build failures, control plane health degradations, etc.
This project should be considered "Step 0" for the overall notifications effort.

Notifications are experimental in 2.15.0 and beta in 2.16.0. We are aiming to complete the GA release in 2.17.0.

This does NOT include IDE notifications.