> What explains the slowdown in IANA timezone database updates?

My guess is that with the increasing dependency on digital systems for our lives the edge-cases where these rules aren't properly updated cause increased amounts of pain "for no good reason".

In Brazil we recently changed our DST rules, it was around 2017/2018. It caused a lot of confusion. I was working with a system where these changes were really important, so I was aware of this change ahead of time. But there are a lot of systems running without too much human intervention, and they are mostly forgotten until someone notices a problem.