The actual story here is that it's slowing down because the lower tail is growing, not because the upper tail is compressing as we hit some kind of upper limit of human longevity. That is: more people are actually dying younger, which has been gradually offsetting other people living longer.
That’s the exact opposite conclusion of this paper.
“Our analysis also revealed that resistance to improvements in life expectancy increased while lifespan inequality declined and mortality compression occurred.”
I wonder if "life expectancy" is really such a useful metric, since it aggregates so many different things together. It's kind of like measuring "aggregate years of human life lived".
It's still useful because it provides some information. Specifically, a change in life expectancy, or a change in the rate of change in life expectancy (as here) means something. We just don't necessarily know what that something is without further study.
It leads to a lot of misunderstandings. For example, lower life expectancy throughout history is far more attributable to higher infant mortality than to adults dying young. But people think everybody died at 45.
