There is no need to mutate public to protected for methods from the implemented interface.

interface Doable
{
    public function do(): void;
}

class Foo implements Doable
{
    public function do(): void {}
}

Incorrect mutation:

class Foo implements Doable
{
-    public function do(): void {}
+    protected function do(): void {}
}

Expected result: no mutation, because it always leads to Fatal error: Access level to Foo::do() must be public (as in class Doable)...

At this point, we don't have information about parent classes/interfaces. It requires implementing a new smart feature with native reflection or BetterReflection.