Unlike virtually every other distro, (https://pkgs.org/search/?q=docbook-xml) Gentoo still maintains some truly primordial versions of docbook-xml in its repos. There are also several packages with hard-dependencies on multiple of these at once. (Looks like there is an issue with eclass inheritance?) I'd suggest for all versions older than 4.5 to be dropped and to make the package otherwise dependent on the (gtk-)doc USE flag. $ equery d docbook-xml-dtd * These packages depend on docbook-xml-dtd: app-accessibility/at-spi2-core-2.42.0 (gtk-doc ? app-text/docbook-xml-dtd:4.3) app-arch/xarchiver-0.5.4.17 (doc ? app-text/docbook-xml-dtd) app-crypt/gcr-3.40.0 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) app-crypt/libsecret-0.20.4-r1 (app-text/docbook-xml-dtd:4.2) (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) app-crypt/seahorse-41.0 (app-text/docbook-xml-dtd:4.2) app-text/asciidoc-10.1.1-r2 (app-text/docbook-xml-dtd:4.5) app-text/gnome-doc-utils-0.20.10-r2 (app-text/docbook-xml-dtd:4.4) app-text/opensp-1.5.2-r7 (doc ? app-text/docbook-xml-dtd:4.1.2) (test ? app-text/docbook-xml-dtd:4.5) app-text/po4a-0.66 (app-text/docbook-xml-dtd:4.1.2) app-text/xmlto-0.0.28-r8 (app-text/docbook-xml-dtd:4.2) dev-lang/orc-0.4.31 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) (gtk-doc ? app-text/docbook-xml-dtd:4.3) dev-libs/appstream-glib-0.7.18-r1 (app-text/docbook-xml-dtd:4.2) (gtk-doc ? app-text/docbook-xml-dtd:4.3) dev-libs/atk-2.36.0 (gtk-doc ? app-text/docbook-xml-dtd:4.3) dev-libs/glib-2.70.4 (gtk-doc ? app-text/docbook-xml-dtd:4.2) (gtk-doc ? app-text/docbook-xml-dtd:4.5) dev-libs/gobject-introspection-1.70.0 (gtk-doc ? app-text/docbook-xml-dtd:4.3) (gtk-doc ? app-text/docbook-xml-dtd:4.5) dev-libs/json-glib-1.6.6-r1 (~app-text/docbook-xml-dtd-4.1.2) dev-libs/libgusb-0.3.10 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) (gtk-doc ? app-text/docbook-xml-dtd:4.4) dev-util/gtk-update-icon-cache-3.24.31 (app-text/docbook-xml-dtd:4.3) gnome-base/dconf-0.40.0 (app-text/docbook-xml-dtd:4.2) gnome-base/gnome-keyring-40.0-r1 (app-text/docbook-xml-dtd:4.3) gnome-base/gvfs-1.48.1-r1 (app-text/docbook-xml-dtd:4.2) gui-libs/libhandy-1.4.0 (gtk-doc ? app-text/docbook-xml-dtd:4.3) media-libs/gexiv2-0.12.2 (gtk-doc ? app-text/docbook-xml-dtd:4.3) media-libs/graphene-1.10.6-r1 (doc ? app-text/docbook-xml-dtd:4.3) net-libs/libsoup-2.74.2 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) net-misc/networkmanager-1.32.12-r1 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) sys-apps/dbus-1.12.20-r4 (app-text/docbook-xml-dtd:4.4) sys-apps/portage-3.0.30-r1 (doc ? ~app-text/docbook-xml-dtd-4.4) sys-apps/systemd-250.3 (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.5) sys-auth/polkit-0.120-r2 (app-text/docbook-xml-dtd:4.1.2) sys-block/gparted-1.3.1 (app-text/docbook-xml-dtd:4.1.2) sys-fs/btrfs-progs-5.16 (app-text/docbook-xml-dtd:4.5) sys-libs/tdb-1.4.6 (app-text/docbook-xml-dtd:4.2) x11-libs/gdk-pixbuf-2.42.6-r1 (app-text/docbook-xml-dtd:4.3) x11-libs/gtk+-2.24.33 (doc ? app-text/docbook-xml-dtd:4.1.2) (doc ? app-text/docbook-xml-dtd:4.3) x11-libs/gtk+-3.24.31 (app-text/docbook-xml-dtd:4.1.2) (gtk-doc ? app-text/docbook-xml-dtd:4.3) x11-libs/gtksourceview-4.8.2 (gtk-doc ? app-text/docbook-xml-dtd:4.3) x11-libs/libSM-1.2.3-r1 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libX11-1.7.3 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXScrnSaver-1.2.3 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXau-1.0.9-r1 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXaw-1.0.14 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXcomposite-0.4.5 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXdmcp-1.1.3 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXext-1.3.4 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXfixes-6.0.0 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXfont2-2.0.5 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXi-1.8 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXinerama-1.1.4-r1 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXmu-1.1.3 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXpresent-1.0.0-r1 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXres-1.2.1 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXt-1.2.1 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXtst-1.2.3-r2 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXv-1.0.11-r2 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libXxf86vm-1.1.4-r2 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/libnotify-0.7.9-r1 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) x11-libs/libwnck-40.0 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) x11-libs/libxcb-1.14 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-libs/vte-0.66.2 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) x11-libs/xtrans-1.4.0 (app-text/docbook-xml-dtd:4.1.2) (app-text/docbook-xml-dtd:4.2) (app-text/docbook-xml-dtd:4.3) x11-misc/colord-1.4.5-r3 (gtk-doc ? app-text/docbook-xml-dtd:4.1.2) x11-misc/shared-mime-info-2.1 (app-text/docbook-xml-dtd:4.1.2) x11-misc/xdg-user-dirs-0.17 (app-text/docbook-xml-dtd:4.3)
When I filed this bug, I apparently had absolutely no clue on how docbook is versioned and how different distributions package it. This report is in fact complete bogus. Sorry for the noise.