We currently do not install any preset files. See URL. This causes systemd to automatically enable all installed unit files when it boots for the first time. This behavior is triggered when /etc/machine-id does not exist. Standard Gentoo behavior is for all daemons to be disabled by default, so we should probably add a preset file to make that reality.
Added in git. https://gitweb.gentoo.org/proj/gentoo-systemd-integration.git/commit/?id=4250e383fe8c8ff946a709e23f90ea5deced3297
Released in v6.