Good questions! 90 days felt like the right balance between keeping enough recent stories readily available and keeping the Stories UI rendering quickly. Stories that fall into this 90 day bucket don’t automatically get archived, they just become hidden, which can be toggled in any done column settings dropdown.
Of course, in hindsight, we’ve realized this is pretty confusing, which is why we’re planning to scrap this old-story-hiding behavior in favor of infinite scrolling for each column. That should get us even better rendering performance without any of the confusion we’ve seen around how this feature works. That should be out the door in the next few weeks.