- 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..