what is the logic behind error - the operand of a 'delete' operator must be optional typescript 4.0

The logic behind of this, is that you need to implement your interface with an optional property like this:

interface Thing {
  prop?: string;
}

function f(x: Thing) {
  delete x.prop; 
}

So the interface's contract won't be broken.

I am not able to realize the logic behind it

The logic as I understand is the following:

Interface Thing is a contract asking to have a (non-null, non-undefined) string prop.

If one removes the property, then the contract is not implemented anymore.

I'm actually surprised that this was not causing error in earlier versions of TypeScript.