/usr/share/xindy/rules/latin-tolower.xdy is in xindy-rules 2.4-1.3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37  | ;; latin-tolower.xdy 			Roger Kehr	 14 Jan 98
;;----------------------------------------------------------------------
;;
;; Maps all latin letters to their lowercase counterpart
;;
(define-rule-set "latin-tolower"
  :rules (("A" "a" :string :again)
	  ("B" "b" :string :again)
	  ("C" "c" :string :again)
	  ("D" "d" :string :again)
	  ("E" "e" :string :again)
	  ("F" "f" :string :again)
	  ("G" "g" :string :again)
	  ("H" "h" :string :again)
	  ("I" "i" :string :again)
	  ("J" "j" :string :again)
	  ("K" "k" :string :again)
	  ("L" "l" :string :again)
	  ("M" "m" :string :again)
	  ("N" "n" :string :again)
	  ("O" "o" :string :again)
	  ("P" "p" :string :again)
	  ("Q" "q" :string :again)
	  ("R" "r" :string :again)
	  ("S" "s" :string :again)
	  ("T" "t" :string :again)
	  ("U" "u" :string :again)
	  ("V" "v" :string :again)
	  ("W" "w" :string :again)
	  ("X" "x" :string :again)
	  ("Y" "y" :string :again)
	  ("Z" "z" :string :again)
	  ))
;; End
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