This file is indexed.

/usr/share/doc/plugins/thunar/INSTALL is in pacpl 5.0.1-1.

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

The actual contents of the file can be viewed below.

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You will have to install the plugin manually using the following:

1)  ./install-gnome.sh

This will modify ${HOME}/.config/Thunar/uca.xml

2) You will also need to edit /etc/pacpl/pacpl.conf and change ZEN_DIR & ZEN_OPTS to 1
   and make sure KDE_DIR & KDE_OPTS = 0

Note: this version can be used with the kde plugin on any system that
supports multiple desktop choices (Slackware for example) if need be. 
You will just need to replace PACPL-Converter by PACPL-Konverter in
your ${HOME}/.config/Thunar/uca.xml
and do the exact opposite of the step 2) above.

Restart Thunar (if already open) to see the changes take effect.