Many thanks for the investigation and explanation @WojciechMazur . I will consider if leaving the LTS behind is viable for my users of the reqT app and lib. Perhaps waiting until 3.8 is released and then rebasing the lib on that is the way to go, given the current situation.
It would be marvelous if there was a way to use a fat jar with the repl of a newer version by somehow give prio to the working stdlib version. Otherwise a fat jar will always nail the compatible repl version to an eventually old one…