Is there a way to flat plane object affecting by proportional edit? As in question title, is there a way to flat plane object affecting by proportional edit without creating new plane object?

here my image :



I don't know how to flat this object except create new plane object or edit position of vertex to z=0 individually.
You can restrict the axis in which you scale to zero.

In this case, select all vertices. press:

S (to activate the scale tool)

Z (to restrict the scaling to the Z axis)

0 to bring all the vertices to the same plane)