Ah OK..., hum that Setting "should" indeed be enabled by Default for all (new) Users when creating an Account on the Forum... All it does (I think) is to "preselect" the "Notify me when a reply is posted" Option when creating a Post in some existing or new Thread where that Setting could always be unchecked at that moment...
But I think the Default and per Post/Thread Notifs should always be 'ON' for all Users, the iMacros Forum is not a "High Volume" Forum and Threads are nearly always "specific" to only one User. I'm practically the only User replying/posting in multiple Threads and I also have it 'ON'.
I'll have to check with the Forum Admin about the Default Value for new Accounts... And Thanks for your Feedback also in "investigating" this part of the "Functionality" for the Forum... (And you now should get some Notif upon this Reply, ah-ah...! )