This file is indexed.

/etc/rcS.d/README is in sysv-rc 2.88dsf-59.3ubuntu2.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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The scripts in this directory whose names begin with an 'S' are
executed once when booting the system, even when booting directly into
single user mode.

The scripts are all symbolic links whose targets are located in
/etc/init.d/ .

To disable a script in this directory, rename it so that it begins
with a 'K' and run 'update-rc.d script defaults' to update the order
using the script dependencies.

For more information see /etc/init.d/README.