I asked for it, but very obliquely and ineffectively within the context of asking for this to apply to groups as well.
The combination would be super helpful, as a way to more gently notify users / groups than the rather sledgehammer-like @mention (which forces people to watch the topic)
That is because it is buried down in the topic share setting at the bottom of the page. I think that most folk would try and share the topic by clicking on the button on the OP so would never see it. Certainly I only found it by accident a few months ago - and fell in love with it immediately!