The WAN port on my home WiFi router in my basement has a directly pingable IPv4 address - I would have thought that was still the most common way people’s houses are connected to the internet?

You could be right. I haven't had one since 2018. ChatGPT says 70% of residential customers worldwide do still have routable addresses so I may have spoken too much from my own experience. I'm sure it depends on where you are, and I don't live in the US, which has a large number of IPv4 addresses per capita. Also IPv6 changes things.