On my system, those units do have unit files, and they're provided by the Plymouth package. I couldn't find any reference to "plymouth-quit-wait" in the systemd

On my system, those units do have unit files, and they're provided by the Plymouth package. I couldn't find any reference to "plymouth-quit-wait" in the systemd source code, so those strings have to be coming from somewhere. Are you sure you don't have a unit file at /usr/lib/systemd/systemd/plymouth-quit-wait.service ?