There isn’t no, brep/plane intersection is one thing that seems to be missing from rhinoscriptsyntax. There is rs.IntersectBreps() but that would require you to make a plane surface bigger than the object to be intersected, and use that to intersect your object. That is one of several workarounds possible.
Brep/Plane intersection is available in RhinoCommon though, so below is a “handmade” function definition that more or less mimics Rhino.AddSrfSectionCrvs. You need to pass the object id and the plane as arguments, it should add the section curves to the document.
import rhinoscriptsyntax as rs
import scriptcontext as sc
#mimics the Rhinoscript method not available in python rhinoscriptsyntax
#returns a list of curve ids of intersection or None if failure
if insec and insec and insec:
crv_ids=[sc.doc.Objects.AddCurve(crv) for crv in insec]
obj_id=rs.GetObject("Select surface or polysurface to intersect",8+16,True)