In my reference filing.

If it's someday maybe, it's on my someday maybe list and any support material is just reference.

If it's old project support that I'll need again, I file into main reference.

If it's old project support that I don't think I'll need again, it goes into Project X folder in the reference filing, and after a year I throw it away that's still in that folder, as I obviously haven't needed it.