is an equivalence with quasi-inverse given by $RQ_ X$.
Proof. Let $V \to W$ be an étale morphism with $V$ affine and $W$ a quasi-compact open subspace of $X$. Then the morphism $V \to W$ is affine as $V$ is affine and $W$ has affine diagonal (Morphisms of Spaces, Lemma 64.20.11). Lemma 72.11.1 then guarantees that the assumption of Lemma 72.11.4 holds. Hence we conclude. $\square$
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