If you ever wanted a way to quickly align all vertices to the same height as a specific vertex, you can do so with Alignment Tool, a simple but useful new Blender add-on developed by 3D Artist known as m_sigepon.
Compatible with Blender 2.8 and above, this tool aligns the positions of selected vertices to any specified coordinates. In Edit Mode, select the vertices of the target object you want to align, open the AlignmentTool that has been added to the Edit tab, get the values of the selected vertices, enter numbers in the coordinates of the direction you want to align, and check the box. Press the OK button to get the result.
