There is no function like this for XF core.
But I used this for years as a mod on VB and I am getting a coder to make it for XF.
Notification Add-on. (not ready yet)
Will notify a usergroup (or several) on all posts in a forum. Admin adds the usergroup/s to the forum settings.
Will notify a usergroup (or several) on all posts on a thread. Admin adds the usergroup/s to the thread settings.
I also want each user to be set to a minimum "membership"for notifications on one category, or three forums. Default setting will be "membership" of a generic intros/ gossip/ support category.
User can change to another category/ or select 3 forums from list/ or add more, in Profile page.
I guess I could ask for this second function to be toggled Enable/ Disable to make it better for other admin choices.