I've never been able to figure it out. Besides an old-fashioned sage, what makes a "thread" (if that is the correct term for it) get deleted? Lack of posts? Too many posts? Or is it simply a matter of time?
I'm mostly referring to the imageboard.
It depends on the settings. The default mode is copied from the Futaba behaviour, and is to delete the oldest thread on the board (no matter its position) when the total number of posts exceeds a set number. Both thread starters and replies are counted as posts.
Wakaba has a number of options to change these behaviours. For instance, you can set it to delete threads when the number of threads exceeds a set number, instead of the number of posts, or to delete a thread when it reaches a certain age, or when the diskspace usage exceeds a certain value. You can also have it delete the last thread instead of the oldest.
Most boards use the default though, and Futaba/Futallaby-based boards only have that mode.
Yes, sage and non-sage posts are counted exactly the same.