Private Notes on Each Network Site

if ( ! is_multisite() ) {
    return;
}

add_action( 'network_site_info_form', function ( $blog_id ) {
    ?>
    <table class="form-table" role="presentation">
        <tr>
            <th scope="row"><label for="wpide_site_note"><?php esc_html_e( 'Notes' ); ?></label></th>
            <td><textarea name="wpide_site_note" id="wpide_site_note" rows="4" class="large-text"><?php echo esc_textarea( (string) get_site_meta( $blog_id, 'wpide_site_note', true ) ); ?></textarea></td>
        </tr>
    </table>
    <?php
} );

// Runs inside site-info.php after WordPress has checked the edit-site nonce and the manage_sites capability.
add_action( 'wp_update_site', function ( $new_site ) {
    if ( ! isset( $_POST['wpide_site_note'] ) || ! current_user_can( 'manage_sites' ) || ! is_network_admin() ) {
        return;
    }

    check_admin_referer( 'edit-site' );
    update_site_meta( $new_site->blog_id, 'wpide_site_note', sanitize_textarea_field( wp_unslash( $_POST['wpide_site_note'] ) ) );
} );

add_filter( 'wpmu_blogs_columns', function ( $columns ) {
    $columns['wpide_site_note'] = __( 'Notes' );

    return $columns;
} );

add_action( 'manage_sites_custom_column', function ( $column, $blog_id ) {
    if ( 'wpide_site_note' === $column ) {
        $note = (string) get_site_meta( $blog_id, 'wpide_site_note', true );
        echo $note ? '<span title="' . esc_attr( $note ) . '">' . esc_html( wp_trim_words( $note, 12 ) ) . '</span>' : '—';
    }
}, 10, 2 );