:unchecked is not defined in the Selectors or CSS UI level 3 specs, nor has it appeared in level 4 of Selectors.
In fact, the quote from W3C is taken from the Selectors 4 spec. Since Selectors 4 recommends using :not(:checked), it’s safe to assume that there is no corresponding :unchecked pseudo. Browser support for :not() and :checked is identical, so that shouldn’t be a problem.
This may seem inconsistent with the :enabled and :disabled states, especially since an element can be neither enabled nor disabled (i.e. the semantics completely do not apply), however there does not appear to be any explanation for this inconsistency.
(:indeterminate does not count, because an element can similarly be neither unchecked, checked nor indeterminate because the semantics don’t apply.)