^ To some extent I agree, that new-user-friendliness is not great in many web forums, but typically this kind of setting is on a row like it is here, at the top of the topic. Sometimes it's in the topic options instead of a separate icon, but it is probably for the best that people aren't auto-subscribed.
Consider that if you are auto-subscribed to every topic you participate in, that even though there are fewer replies to topics as they age, over years of posting you will be getting more and more topic replies. There was a time when I was getting well over 50 topic replies a day due to so many years I had posted in forums and it was just too much, so one by one I had to unsubscribe unless I wanted to nuke all replies from the entire forum. In an automotive context that sometimes made sense, for example if it was for a vehicle forum and I no longer owned the vehicle.
I've registered at probably over 500 forums the last 20 years and some of them have automatically set the default notification to "do not subscribe." So I make a few posts, but thinking I didn't get any replies because I didn't get an email notifying me of them, I don't bother visiting that forum again. I can't imagine why forum administrators wouldn't make the default notification to "instant" and why posters wouldn't want that. I mean, why post a comment or question if you don't want to see the reply. And as for flooding your email with notifications, most forums only send ONE email per thread until you log back in to the thread, but of course, they don't have that here either. EVERYtime there is a reply in the thread, you get an email. Why can't THAT be fixed?