 Solution

The defining restriction is the reality conditionFor a non-Abelian gauge group, write the vector superfield in a Hermitian generator basis with real superfield coefficients. This is a condition on the full superfield, not a chirality constraint. It relates conjugate component coefficients and makes the vector and $D$ auxiliary field real. Wess-Zumino gauge is a further supergauge transformation choice removing redundant components; it is not the defining condition for a vector superfield.