/usr/share/fish/functions/export.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 4 5 6 7 8 9 10 11 12 13 14 15 | function export --description 'Set global variable. Alias for set -gx, made for bash compatibility'
    if test -z "$argv"
        set
        return 0
    end
    for arg in $argv
        set -l v (string split -m 1 "=" -- $arg)
        switch (count $v)
            case 1
                set -gx $v $$v
            case 2
                set -gx $v[1] $v[2]
        end
    end
end
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