Install this version:
emerge -a =app-emacs/s-9999
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =app-emacs/s-9999
Or alternatively:
emerge --autounmask-write -a =app-emacs/s-9999
# Copyright 1999-2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
NEED_EMACS="24"
inherit elisp git-r3
DESCRIPTION="The long lost Emacs string manipulation library."
HOMEPAGE="https://github.com/magnars/s.el"
EGIT_REPO_URI="https://github.com/magnars/s.el.git"
KEYWORDS="~amd64 ~arm ~arm64 ~ppc64 ~x86"
LICENSE="GPL-3+"
SLOT="0"
SITEFILE="50${PN}-gentoo.el"
DOCS="README.md"