Oort's amalgamation conjecture for Newton polygons of curves

Let $p$ be a prime and let $C$ and $C'$ be smooth proper irreducible curves of genera $g$ and $g'$, with Newton polygons $P$ and $P'$, respectively. A Newton polygon is determined by its multiset of slopes, counted with multiplicity; the amalgamation of $P$ and $P'$ is the Newton polygon whose slopes are the disjoint union of the slopes of $P$ and $P'$. Oort's amalgamation conjecture. There exists a smooth proper irreducible curve $C”$ of genus $g+g'$ whose Newton polygon is the amalgamation of $P$ and $P'$. This conjecture gives guidance on which Newton polygons can occur for curves in characteristic $p$: it predicts that combining the Newton polygons of two curves can be realized by a single curve of the summed genus. The paper cites Oort's formulation as Conjecture 8.5.7; the source provides no resolution status.