The specified init.d script has no systemd equivalent and the package ships other units.
This typically occurs when a maintainer missed script when adding systemd integration, or a new init script was added in a new upstream version.
Systemd has a SysV init.d script compatibility mode. It provides access to each SysV init.d script as long as there is no native service file with the same name (e.g. /lib/systemd/system/rsyslog.service corresponds to /etc/init.d/rsyslog).