/usr/share/fish/completions/echo.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 | complete -c echo -s n --description "Do not output a newline"
complete -c echo -s s --description "Do not separate arguments with spaces"
complete -c echo -s E --description "Disable backslash escapes"
complete -c echo -s e --description "Enable backslash escapes"
complete -c echo -s h -l help --description "Display help and exit"
complete -c echo -u
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