What is the current estimated diameter of the Internet?

Maximum (worst-case) RTT edge-to-edge?

Most public latency data is now edge-to-cloud, not edge-to-edge.  Cloud engineers have done a great job, and edge-to-cloud less than 1-sec RTT.

Where have the long-slow pipes gone?

https://www.cloudping.co/grid

https://learn.microsoft.com/en-us/azure/networking/azure-network-latency?tabs=Americas%2CWestUS

https://www.verizon.com/business/terms/latency/

The RTT across geostationary satellites has remained stubbornly constant we just don't use them unless there's no other alternative. A colleague sent me a trace the other day while flying from hawaii to los vegas that was otherwise performant but still incurs the overhead.