You can use featureEdges. You get both upper and lower nodes, but it's not difficult to distinguish them
You can also check for nodes lying on the plane, as I suggested in yhe answer to your previous question here
nodes = importdata("NODES.mat");
faces = importdata("FACES.mat");
P1 = [28.9646, -21.3886, 97.3614];
P2 = [17.2506, -22.7154, 62.1242];
figure
plot3(nodes(:,1),nodes(:,2),nodes(:,3),'k.','Markersize',2);
hold on
trimesh(faces(:,:),nodes(:,1),nodes(:,2),nodes(:,3),'EdgeColor','k','Linewidth',0.1,'Facecolor','b','FaceAlpha',1)
plot3(P1(:,1),P1(:,2),P1(:,3),'r.','Markersize',27);
plot3(P2(:,1),P2(:,2),P2(:,3),'r.','Markersize',27);
% hold off
axis equal
TR = triangulation(faces,nodes);
E = featureEdges(TR,pi/3);
P = nodes(E(:),:);
plot3(P(:,1),P(:,2),P(:,3),'ro','MarkerFaceColor','r')
