The person above made it clearer. Default will ask the compiler to add a {} for you.
If your class holds some resources, the virtual destructor should not be empty,
needs to do some cleanup work. For example:

class A{
public:
    virtual ~A(){
        free(handle);
        delete ptr;
    }
    
    someFilehandle* handle;
    somePointer* ptr;   
};

If A is used as a parent class, then its destructor should be declared as virtual as much as possible to avoid
the following situations:

class B : public A {
public:
    //some declaration
    virtual ~B() {}
};

//假使你获得了或者创建了一个A*类型的指向一个B*对象
{
    A* a = new B();
    delete a;//如果A的构造函数不是virtual，那么这个时候
             //就是一个未定义行为，B的析构函数多半不会调用
             //导致可能的资源/内存泄漏。
}

No, you have explicitly defined the default destructor as your destructor. If you want to customize it, it will not be default

=default is not empty, but the default behavior is used, or the destructor of the member is called. This is not essentially different from writing {} directly.

virtual ~Deconst()=default；
virtual ~Deconst() {}