Exact polynomial-degree conjecture for the stable homology of IA-groups

Let $F_n$ be a free group of rank $n$, and let $IA_n$ be the kernel of the canonical epimorphism ${\rm Aut}(F_n)\twoheadrightarrow GL_n(\mathbb{Z})$. For each $d\in\mathbb{N}$, let ${\mathcal{H}}_d(IA;\mathbb{Z})$ denote the associated stable homology object, and let ${\mathcal{P}ol}_r(\mathbf{S}(\mathbb{Z}),\mathbb{Z})$ denote the category of polynomial objects of degree at most $r$. Exact-degree conjecture. For every $d\in\mathbb{N}$, ${\mathcal{H}}_d(IA;\mathbb{Z})$ belongs to ${\mathcal{P}ol}_{3d}(\mathbf{S}(\mathbb{Z}),\mathbb{Z})$ but not to ${\mathcal{P}ol}_{3d-1}(\mathbf{S}(\mathbb{Z}),\mathbb{Z})$. This is presented as a further conjecture beyond strong polynomiality, and the source does not indicate a proof or resolution.