Each of the 4 status of Incident and Maintenance has a default message.
for example, default message "In Progress" maintenance status is "Scheduled maintenance is still in progress. We will provide updates as necessary."
How can I change this default messages?
For a start, you might want to check where this notification is configured.
Some verification suggestions are:
Access the project settings and click on "Customer Notifications", in this place it separates the notifications sent to customers. If you have any custom notifications with the comment you indicated, this is where you can change or create a new one.
If you don't identify it in the "Customer notifications", you can check the flow, if in the transition there is no post function that makes this notification.
Or you can even check the automation rules if there's anything set up to do so.
By locating where this notification is made, you can edit it.