Using get_query_var() from a plugin

by

Hook into parse_query instead:

add_action( 'parse_query', function() {
    add_rewrite_rule( 'go/([^/]+)/?$', 'index.php?redirect_to=$matches[1]', 'top' );
    add_rewrite_tag( "%redirect_to%", '([^&]+)' );`
} );

Reason: init is too early, query_vars haven’t been set yet. wp_head is too late, no can redirect at that point.