Given a strongly local Dirichlet space and $λ\geq 0$, we introduce a new notion of $λ$--subharmonicity for $L^1_\loc$--functions, which we call \emph{local $λ$--shift defectivity}, and which turns out to be equivalent to distributional $λ$--subharmonicity in the Riemannian case. We study the regularity of these functions on a new class of strongly local Dirichlet, so called locally smoothing spaces, which includes Riemannian manifolds (without any curvature assumptions), finite dimensional RCD spaces, Carnot groups, and Sierpinski gaskets. As a byproduct of this regularity theory, we obtain in this general framework a proof of a conjecture by Braverman, Milatovic, Shubin on the positivity of distributional $L^q$-solutions of $Δf\leq f$ for complete Riemannian manifolds.