15Teds it can be done, but there's no extension that this time that lets you selectively configure it. You can make the changes to the default settings/notifications from the extend.php file.
One method is to hard-code a value during user signup, like https://discuss.flarum.org/d/18607-friendsofflarum-default-user-preferences does for followAfterReply.
Another option is to use the preference extender and change the default value of the preference. The only example I have for that is unfortunately outdated here (it uses the event that has now been replaced by an extender)
The first method has the "benefit" of only applying to new members, while the second will actually change the setting for all users who never changed it.
