This is the same problem as "why aren't the docs up to date" and asking for the "why" is going to break down in the same place.

I think there might be ways to mostly sidestep this, though. Docs that look more like changelogs than "docs." Store them reverse chronologically so someone new can either look back at recent history or try to jump in to when a particular feature was originally added. Lean in to there being a series of "whys" for the code.