I can confirm the ojdbc6-11.2.0.1.0.jar that came with Oracle 11.2.0.1.0 can work with Oracle 19c if you set SQLNET.ALLOWED_LOGON_VERSION_SERVER=11 on the server side. The same should hold for ojdbc6-11.2.0.4.jar.

According to OJDBC FAQ under the question "Oracle JDBC releases Vs JDK versions" the last ojdbc6 came with Oracle 12.2 but I don't have experience with this one and I can't find it in Maven repository.