[clappski]

> But wouldn't the ability to restrict arbitrary changes to a struct still be important?

Typically immutability in C++ is at the interface scope rather than structural, e.g. prefer this

    struct MyType {
        int x_;

        MyType( int x ) : x_( x ) {}
    };
   
    void do_the_thing( const MyType& mt ); // mt.x_ = 1; doesn't compile, mt is const

over this

    struct MyType {
        const int x_;

        MyType( int x ) : x_( x ) {}
    };
   
    void do_the_thing( MyType mt ); // mt.x_ = 1; doesn't compile, x_ is const