Two years ago I did something similar. Plotting the surviving lines of code in the OpenBSD code base across commits:

https://twitter.com/mulander/status/809120593606049792

Nice plot. Interestingly, it seems that by far, relatively, most code from 1999 was removed. What did they add in that year, which got removed then (around 2014)?

not op, and not 100%/authoritative, but I can think of some things:

- adding/refactoring locking for improved SMP support

- dropping older architectures (VAX, etc)

- dropping older protocols/servers (e.g ISDN, decnet sorts of things, obsoleted proto-IPv6 versions)

- dropping/refactoring systrace

- rewriting or dropping various network routing daemons (apache HTTPD 1.3 removed from tree at this time)

- libressl replacement at this time

see also: https://en.wikipedia.org/wiki/OpenBSD_version_history

relatedly, in the openbsd world, Ted Unangst is well known enough for doing old/unused code audit+removal that there is a slang verb 'tedu' (his handle, usage e.g. "it got tedu'd") which means basically zapping old stuff. See 1st comment in the twitter thread..

Y2K mitigation code

Monotonic functions are boring to look at. I'd rather see its derivative, probably smoothed out a bit.

you can use Gource to get a graph like this: https://youtu.be/iZjvVxbM3kY