In the Find Data dialog, there is an option to show the frustum defined by a Select Cells Through or Select Points Through operation. A checkable button controls the visibility of the frustum, and is highlighted blue in the screenshot below.
That was useful way way way back during development of the feature, and a good way to make illustrations for the books and such. I can’t really think of a practical use for it today though.
I completely agree with Mathieu with regards to classes.
The one reason I can think of for removing the frustrum code is that ParaView is already a hugely complex program. Removing unneeded algorithms and code simplifies said code.