The matroid (basis) polytope of a matroid on n elements lives in R^n, and is the convex hull of the indicator vectors of the bases.
For uniform matroids, the basis polytope is precisely the hypersimplex:


In order to obtain an actual polytope object in M2, one must take the convex hull of the columns of the indicator matrix, which is provided by either the Polyhedra or OldPolyhedra package:



The GelfandGoreskyMacPhersonSerganova (GGMS) theorem characterizes which polytopes are basis polytopes for a matroid: namely, each edge is of the form $e_i  e_j$ for some $i, j$, where $e_i$ are the standard basis vectors.
The object basisIndicatorMatrix is a method function.