Install this version:
emerge -a =app-emacs/df-mode-20050509-r1
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =app-emacs/df-mode-20050509-r1
Or alternatively:
emerge --autounmask-write -a =app-emacs/df-mode-20050509-r1
# Copyright 1999-2019 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=7
inherit elisp
DESCRIPTION="Minor mode to show space left on devices in the mode line"
HOMEPAGE="https://web.archive.org/web/20061001221337/http://www.coli.uni-saarland.de/~fouvry/software.html
https://www.emacswiki.org/emacs/DfMode"
# taken from http://www.coli.uni-saarland.de/~fouvry/files/df-mode.el.gz
SRC_URI="mirror://gentoo/${P}.el.bz2"
LICENSE="GPL-2+"
SLOT="0"
KEYWORDS="amd64 ~ppc x86"
SITEFILE="50${PN}-gentoo.el"