I decided to give the webpacker 6.0.0 prerelease a try, to get finally rid of python 2.7. I created a new application and then followed the upgrade procedure webpacker/ 6_0_upgrade.md And it did not work.
This is a change between 5.2.1 and 6.0.0pre, and it is not mentioned anywhere. I now have read all docs, two times and a third time, and I do not find a hint about what this could be about.
So please, can somebody give me any pointer, links, keyword, whatever where I can read about this scheme, what it is meant to be, and how it is correctly handled?