add_filter(
    'activitypub_extract_mentions',
    function (
        array $mentions,
        string $content,
        WP_Post $post
    ) {
        // Your code here
        return $mentions;
    },
    10,
    3
);

function prefixed_filter_callback( array $mentions, string $content, WP_Post $post ) {
    // Your code here.
    return $mentions;
}
add_filter( 'activitypub_extract_mentions', 'prefixed_filter_callback', 10, 3 );