This file is indexed.

/etc/grub.d/41_custom is in grub-common 1.99-21ubuntu3.19.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#!/bin/sh
cat <<EOF
if [ -f  \$prefix/custom.cfg ]; then
  source \$prefix/custom.cfg;
fi
EOF