Office Insider 以外でも利用可能になったようです。
Task Summary Name field
With long lists of tasks, it can be difficult to know what a task is indented under in the overall project plan. TheTask Summary Name field is a read-only field that shows the name of a task's summary task. Adding this field as a column in your Task view can help clarify your project's structure. To add this field, right-click the title of a column (to the right of where you want to add the field), selectInsert Column, and then chooseTask Summary Name from the drop-down list.