The default view is "show threads from the last 100 days". Because there have only been 30 threads in the last 100 days that are active, it only shows 30. Go to the bottom and change to "view all" and you can find all the older ones. Alternatively, I can change the default setting to "show all" if enough people want it like that.
The thread you linked to was either deleted or merged. It isn't related to the issue of thread date cutoff. When I search for "Assumptive-Fingertrick-Notation" I get linked to the thread it appears to be merged into. In the event you get a "thread doesn't exist", it means it was merged or deleted, and it would be worth searching for the title as you'll likely find it that way if it was merged. Hope that helps.