List of free notification stuff for developerππ½
If you're looking for all programming tools around notification (not just free). Check this page List of notification Programming Tools

Novu
The ultimate library for managing multi-channel transactional notifications with a single API.
Tag: notification
open source (self hosted)

Engagespot
Send transactional Email, SMS, In-app, Whatsapp, Push, Slack notifications using a single API.
Tag: notification, communication, messaging
Free for up to 100 monthly active users.