Why are varactors unspecified at 0V?
Because varactors are diodes.  The special thing about a varactor diode is that it is deliberately designed to harness how the depletion region size grows and shrinks with reverse applied voltage.  That changes the capacitance between the terminals.  You apply a DC reverse voltage to set the capacitance, then use that with a small AC signal.  The DC bias can change, which changes the capacitance.

However, all this stops working if the diode becomes forward biased.  This means the DC bias plus the peaks of the AC signal must still be negative.  To use a varactor as intended, you always need some negative DC bias.  It's also good to stay out of the region right at 0 bias because it's less linear and small changes in voltage have a larger effect.  As a result, nobody cares what the diode does with less than -1 V bias, since you don't want to use it that way.