Install this version:
emerge -a =app-emacs/undo-tree-0.8.2
# Copyright 1999-2022 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
NEED_EMACS="24"
ELPA_CONTAINER_KIND=tar
inherit readme.gentoo-r1 elpa
DESCRIPTION="Undo trees and visualization"
HOMEPAGE="http://www.dr-qubit.org/undo-tree.html"
SLOT="0"
KEYWORDS="amd64 x86"
SITEFILE="50${PN}-gentoo.el"
DOC_CONTENTS="To enable undo trees globally, place '(global-undo-tree-mode)'
in your init file."
DEPEND="app-emacs/queue"
RDEPEND="${DEPEND}"