I request a feature to easily select multiple extents at once to delete them.
My workflow in a multi biome landscape is to make one larger extent which is used for building the terrain.
The smaller extents are used to focus on a single biome at a time.
The need to activate an extent before you can delete can be quite slow when you have a lot of devices in the project!
I suggest something like a list where you can check or uncheck extents, then click delete.
OR
The extent is not activated/rendered until you click OK in the Project Settings.