> I propose (not for the first time) that similarly concatenating an unknown number of iterables or dicts should be possible via comprehensions:

I'm really happy to see this feature proposal come up again!  Every time, it seems this feature is even more intuitive to even more people, and I'm excited to see so many +1s compared to the -1s.  I hope it will be accepted, if not this time, then one day :)

It was considered and rejected in PEP 448. What was changed since? What new facts or arguments have emerged?

Just for the record, both Joshua and I worked on implementing PEP 448.  We both wanted this feature because we felt it was consistent.  However, we wanted to maximize the probability that PEP 448 was accepted, so we chose to defer this feature.