Is there a way to implement sectionIndexTitles for diffable data source? Or maybe even mix or use diffable datasource and the classic tableview datasource together to show the index titles?

Not sure if I understand you correctly, but maybe just implementing

    func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
        let cell = tableView.dequeueReusableHeaderFooterView(withIdentifier: "Cell")

        //Configure the cell

        return cell
    }