Is it safe to use GameObject as a Dictionary key?

No there shouldn't be any issues with the Dictionary, That's because as you know a C# managed object can not be destroyed, only the native part. Your managed object will turn into a fake null object but that shouldn't cause any problems. The UnityEngine.Object class actually overrides GetHashCode and simply returns the instance ID of the gameobject. However this ID is stored in the managed object itself and it stays the same when you destroy the object. So since the hashcode stays the same from the Dictionary point of view it only deals with an object reference.

The overloaded == operator of UnityEngine.Object is not used inside the Dictionary since operators are statically linked. So inside the generic Dictionary it will use the default == operator. The Dictionary actually performs a null check on the key when you get / set a value. Since the managed object is still a valid C# object and the == operator actually checks the real reference it just works the same.

Unity also overrides the virtual Equals method to implement the fake null behaviour. Though a direct comparison sill works the same so go.Equals(go) will return true even for fake null objects.

Note that this actually holds true for any UnityEngine.Object derived type which covers nearly every object type that is shipped with the Unity engine (GameObject, Component, Mesh, Material, ...)