The rcS init.d script has no systemd equivalent.
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).
Services in rcS.d are particularly problematic, because they often cause dependency loops, as they are ordered very early in the boot sequence.
Refer to https://wiki.debian.org/Teams/pkg-systemd/rcSMigration for details.