I would not want the code that was tested and put to production to start producing warnings or even crashing when not updated by a certain date. For whatever reason the schedule may slip, the new version be postponed and then what? Will you release a quickfix that just changes the dates? And hope you did not forget any or change the computer clock before you run the test to make sure?
If the Devel::Deprecate knows to only make fuss within the test suite, great, if it's a timebomb within production code, no thanks.