WordPress - How to sanitize multi-line text from a textarea without losing line breaks?

Since Wordpress 4.7.0

Use sanitize_textarea_field instead of sanitize_text_field

A more elegant solution:

update_post_meta(
    $post_id,
    'message',
    implode( "\n", array_map( 'sanitize_textarea_field', explode( "\n", $_POST['message'] ) ) )
);

Use sanitize_text_field if you want to sanitize text field.