It's become much easier to do this with the advent of tiled vector data and libraries like https://github.com/maplibre/geojson-vt
Here is an example using Mapbox GL - https://demos.mapbox.com/boundaries-explorer/?country=US&lay...
Thank you for the links. The postcode project I mentioned was 5 years ago. So this definitely didn't exist back then. Great to see such progress with these libraries.
Quite something to see this in a browser on my phone, rendered that easily. I can clearly remember how hard this was back then.