Only finitely many modular Jacobians are supersingular modulo a given prime

In this paper, we study supersingularity of modular Jacobians $J_0(N)$ and abelian varieties $A_f$ of ${\mathrm{G}\mathrm{L}}_2$-type from both the vertical perspective (where the prime $p$ is fixed, and the level $N$ varies) and the horizontal perspective (where the newform $f$ is fixed, and the prime $p$ varies). Vertically, we prove that $J_0(N)$ is supersingular modulo a given prime $p$ for only finitely many levels $N$. Horizontally, we show that for sufficiently large $p$, the reduction of $A_f$ modulo $p$ is supersingular if and only if it isogenous to a power of a supersingular elliptic curve.