Modern version control systems (Git, Hg, etc.) do a great job of storing file commits. They're very "flat," however. A file is a file is a file, and there's no

Modern version control systems (Git, Hg, etc.) do a great job of storing file commits. They're very "flat," however. A file is a file is a file, and there's no good mechanism for storing tightly-linked documentation about the evolution of a project. Commented-out code is just one ad hoc way developers try to tack on that kind of information (often in clumsy, likely-to-be-obsoleted-or-forgotten ways).