Upstream has deprecated the `udevadm` subcommand `hwdb` in favor of a tool called `systemd-hwdb` four years ago and retains `udevadmn hwdb` for backwards compat [1] OTOH sys-fs/udev comes without `systemd-hwdb` and while the deprecated `udevadmn hwdb` (still) works, it isn't documented. If Gentoo wants to keep the "for backwards compat" version, at least there should be mention in the manpage and the online documentation. Or, perhaps, ship systemd-hwdb? [1] https://github.com/systemd/systemd/blob/5639df9a80a828f66ac1ab703dd4fe21a0fc1d19/src/udev/udevadm-hwdb.c#L558
sys-apps/systemd-utils[udev] installs systemd-hwdb.