Using an Arcade expression:
I can get a count of a polyline's vertices by looping through the paths property:
var line_geo = Dictionary(Text(Geometry($feature)));
var paths = line_geo['paths'];
var vertex_count = 0
for (var path_idx in paths){
for (var vert_idx in paths[path_idx])
{
vertex_count++
}
}
return vertex_count

That code works, but the vertex count is a densified vertex count, which isn't what I want...I was expecting the vertex count to be 3 vertices, not 35 vertices.
Could ESRI give us a way to get the true vertex count? For example, a pointCount() geometry function.