This file is indexed.

/etc/X11/Xsession.d/35x11-common_xhost-local is in x11-common 1:7.7+1ubuntu8.

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

The actual contents of the file can be viewed below.

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# This file is sourced by Xsession(5), not executed.

# If xhost (from x11-xserver-utils) is installed, use it to give access 
# to the X server to any process from the same user on the local host.
# Unlike other uses of xhost, this is safe since the kernel can check 
# the actual owner of the calling process.

if type xhost >/dev/null 2>&1; then
  xhost +si:localuser:$(id -un) || :
fi