PORTDIR
From Gentoo Wiki
The PORTDIR variable was[1] used to point to the main package repository hosted on the system. It has since been deprecated in favor of the location attribute (whose default value is /usr/portage) of the Gentoo repository configuration inside /etc/portage/repos.conf.
The current value of the location attribute can be obtained by using portageq:
user $portageq get_repo_path / gentoo/usr/portage