Hello @Kiera_Wilson
As far as I know, Adobe Target have a built in notification system which is available inside the Adobe Target product only like below

However this is not available for external system where you wanted to send notification as you mentioned.
The other way is possible through using Adobe Target Admin API and very specific to notification - Update Activity state
You can integrate the above APIs with your other 3rd party system to get the automatic notification when journeys are activated and deactivated.
Hope this helps.
Thanks