During maintenance ATS can indeed not be used. Schedules are disabled and running tests will be canceled when maintenance begins. Schedules that have not been started yet will be picked up after the app is back online.
ATS will show an alert some time before ATS will go down in a similar way as the alert below:
Which Mendix versions are supported by the ATS version can be found in either the release notes or in this documentation: https://docs.mendix.com/ats/general/compatibility.
Thank you for the quick response