The lib path is /automation/jython/lib. Just put your helper libs to this folder. More details can be found here
The related breaking change was during the openhab 4.2 release. Also the change from automation/jsr223 to automation/jython was happens there.
But maybe its time to give python3 a try :-). The only bad news there is, that the helper libs, you are talking, are not working there anymore. As a replacement you have a very lightway new helper lib.