Once you post in a thread, it appears in your Updated Topics at top-right whenever there's activity in the thread.
There are a few threads where I'm not even reading new posts, for whatever reason: is it possible to opt out of a thread I'm already in?
This is kinda hidden, but you can do it like this:
Excellent, thanks.
Philosophical observation: I'll never know if that worked 
Did you see this? lol
pYro_65:
Did you see this? lol
Yeah but this is isn't a thread I had opted out of
But I have now opted out, so if you post again I'll check back from time to time to see if my opt-out worked.
So, here's another post in this thread.
PaulS:
So, here's another post in this thread.
Nope it doesn't work. If you click not following the box title changes but if you leave and come back, or even just refresh the thread, it reverts.
Unless there's a "save settings" that I'm missing?
No, and possibly yes. It only has effect if you do not go back to the thread. By viewing the thread you are not "not following", and the forum is simply helping you not not follow the forum.
pYro_65:
By viewing the thread you are not "not following", and the forum is simply helping you not not follow the forum.
You're lucky this is a programming forum. If we were macrame exponents, or amateur brain surgeons, that would have made no sense at all.
Luck has no part in that.
That post was made because of the nature of this forum.
Anyway, it doesn't work. I had definitely set Not Following, hadn't returned to the thread, but got it in my Updated Topics when #9 was posted.
I'll Not Follow now again just to make sure though.