It is simple to unwrap a deformed plane like in your image, back to a square UV map.

In face select mode, select all the faces with A and make one of the faces active.
Then press U > Follow Active Quads and then choose Even and press OK.

This will give you a even square UV layout.

It is simple to unwrap a deformed plane like in your image, back to a square UV map.

In face select mode, select all the faces with A and make one of the faces active.
Then press U > Follow Active Quads and then choose Even and press OK.

This will give you a even square UV layout.