On my setup (profile default/linux/amd64/23.0/split-usr/hardened/selinux, OpenRC 0.54.2) the plymouth.so from the emerged package is placed in the /usr/lib64/rc/plugins folder. This is not then picked up by OpenRC. Manually adding a symlink /lib64/rc -> /usr/lib64/rc fixes the issue, as does downgrading to v0.1.3, which installs the so file in the /lib64/rc/plugins directory. Likely a bad interaction with the split-usr profile.