Create a Setter Only in Swift

Well if I really have to, I would use this.

Swift compiler supports some attributes on getters, so you can use @available(*, unavailable):

public subscript(index: Int) -> T {
    @available(*, unavailable)
    get {
        fatalError("You cannot read from this object.")
    }
    set(v) {
    }
}

This will clearly deliver your intention to the code users.