Wanhao made a change and no longer put on a lower z axis endstop switch.
If you are using the correct dimensions in your Wanhao Maker profile the lack of this endstop switch should not be an issue as the slicer will warn you of tall models that would allow the build plate to crash into the bottom of the frame.
You can edit your Wanhao Maker profile by going to EDIT > SETTINGS on the Wanhao Maker user interface.
Change the maximum height on Duplicator 5S to 575 mm. On Duplicator 5S Mini that value should be 175 mm