Version: Clarity 15.7.1, 15.8, 15.8.1, 15.9, 15.9.1
The Modern UX Task Timeline view is not configurable in 15.9.1 and lower. This includes:
The Board and Grid views can be customized.
The Timeline view is configurable in the latest versions of the product. See New Timeline Layout for the Tasks Module
Fields in the Task timeline view are now able to be edited. See Edit Fields in Timeline Layout for Tasks