It is an old feature to emulate constructor call from D1 age.

struct S {
    int num;
    static S opCall(int n) {
        S s;
        s.num = n;
        return s;
    }
}
void main() {
    S s = 1;
    // This line is translated to:
    // S s = S.opCall(1);
}

But in D2, "implicit constructor call" feature properly supports such the initializing syntax.

struct S {
    int num;
    this(int n) {
        this.num = n;
    }
}
void main() {
    S s = 1;
    // This line is translated to:
    // S s = S(1);
}

Therefore, we can deprecate/remove the problematic opCall feature in initializing.