Enable notifications for deployments in Chat Notification Services
Currently, the Slack notifications settings allow for events such as Push, Issue, Merge, Note, Pipeline, etc, but it does not have a specific event for new deployments.
EDIT: This seems to be related to other chat services as well, not just slack.
Create a new notification event for deployments (either manual or automatic, successful or failed), for integration with the Slack notifications webhook.
Links / references