So I just noticed this feature.
Personally I would be much more inclined to use it,
if it also showed NEW threads you've never seen before.
That gives you a chance to notice subject you're interested in.
If you don't ever try to read it, it won't be displayed again.
Doesn't seem like large difference in # of threads shown at one time.
"IMHO"
The website does that with the Expand/Flatten mode.
If you start on the main messageboard page in the "Expand" mode, you can see a list of all the forums and subforums.
There's a little arrow to the left of each forum name. Clicking it toggles it to point down or point right.
If you leave it pointed down, all the threads in that forum will be visible on the main page. If you leave it pointing right, the main page will not show any threads from that forum (or subforum)
Now, if you toggle back to the "Flatten" mode, you'll see all the threads from all of your chosen forums listed with the most recent on top.
Each thread also has a little null symbol over on the far right. Click that null symbol will hide any individual threads you don't want to see again.
The Flatten/Expand button is a toggle, so if you want to view the "Flattened" view, you'll see the "expand" button ready to be clicked.
Doesn't sound like what I'm talking about at all.
Which has nothing to do with (sub) Forum specific views.
(although if implemented, (sub) Forums with few new threads would often not display anything even without arrow being toggled 'closed')
Also has opposite process re: not seeing threads after first time.
What you describe needs user interaction to NOT see such a thread.
What I describe needs user interaction (reading thread) to see it again.
But reading (visiting) a thread is not an imposition for thread you are interested in seeing again.
Needing to take action on threads you know you are uninterested in IS extra imposition.
It is ridiculous for people to click that X for every thread they are disinterested in.
Having an option to no longer display thread you once checked out but no longer want to see is fine.
I'm just addressing how the Focus ability can work in way actually useful to me.
If I can have chance to see new threads at least once, I would be happy using it most all of the time.
Without that, I wouldn't.
You get what you're asking for by first setting up using the Flatten mode and Defocused mode.
Once you've scanned all the new posts since you last visited, and have clicked to read the ones you find intriguing, you can toggle to focused mode and only see the threads you've previously read.
So it's a two-step process
1) scan all the new threads and open the interesting ones
2) toggle to focused to see what has happened in the threads you've previously viewed.
They just did a big site recoding, so it's not likely that they'll take on another round of revisions any time soon.
If you want to use the site the way you describe, there's a work-around.