It's very common in old software and video games. Pre-97 office did that though with more formalisation than most (the binary format is now called Binary Interchange File Format, or BIFF, and is relatively generic, at the time MS almost certainly had utility libraries for working with BIFF).

This was a common source of portability issues for game saves (as well as blowing your saves on updates), as they'd commonly just blit internal data structure, with no formalised interface.