in_footer: gives syntax error

As of WordPress 6.3, the in_footer parameter is now bundled into the args array parameter (source).

The correct syntax for the wp_enqueue_script() function is now as follows (leaving out the named function parameter as it not officially supported by WP core):

wp_enqueue_script( string $handle, string $src="", string[] $deps = array(), string|bool|null $ver = false, array|bool $args = array() )

In your case, you would write your function as follows:

$args = array(
    'in_footer' => 'true'
);

wp_enqueue_script('swiper-script', 'https://unpkg.com/swiper@8/swiper-bundle.min.js', [], false, $args);

Your revised wp_enqueue_script() function you now rely on works because there is still backwards compatibility for the in_footer parameter as @Howdy_McGee points out, (see here), but only if you’re not specifying in_footer: true and instead just true since the parameter is removed from the function declaration.