It provides visual feedback for links that have "focus" when navigating a web document using the TAB key (or equivalent). This is especially useful for folks who can't use a mouse or have a visual impairment. If you remove the outline you are making your site inaccessible for these people.
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... If you must remove it, provide alternative styling.
Thanks! I'll consider adding a warning for this in a future update.
UPDATE: The next v14 update will inform (or warn if accessibility checking is enabled) when the outline is removed from "a" (set to 'none' or to a 0 width).