/usr/share/fish/functions/setenv.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | function setenv --description 'Set global variable. Alias for set -g, made for csh compatibility'
set -gx $argv
end
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