Not avionics, but I've spent most of my career in medical devices. A project's Design History File would indeed have the detailed design documentation of the system. The problem would be that word "detailed." The relevant standards state that the documentation should be sufficient for a normally-skilled engineer to recreate the (software) system, but how stringently that is applied can be very hand-wavy. And then there's the problem of drift where the software changes and the docs are updated, but perhaps not as precisely as they should be, and different people have different levels of documentation rigor. Well, you can see how it ends up 20 years later.

I'm actually dealing with exactly this problem on an old project right now :-(