A bit of light googling turns up several sources asserting that the model structure on simplicial commutative algebras over a ring is left proper (for example, 2.9 in Charles Rezk's paper Every homotopy theory of simplicial algebras admits a proper model). Does a proof of this fact occur anywhere in the literature?
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For simplicial commutative rings this is proved in Lemma 3.1.2 of Schwede's “Spectra in model categories and applications to the algebraic cotangent complex”, and the proof there immediately extends to algebras. 

