I've done extensive work on improving the Postgres B-Tree code, over quite a number of releases. I'm not aware of any problems with high-insert workloads in particular. I have personally fixed a number of subtle issues that could lead to lower space utilization with such workloads [1][2] in the past, though.

if there's a remaining problem in this area, then I'd very much like to know about it.

[1] https://www.youtube.com/watch?v=p5RaATILoiE [2] https://speakerdeck.com/peterg/nbtree-arch-pgcon