[pjc50]

> Today, finding out that 2 cell phones have been in proximity requires computationally expensive analytics on the server side.

No, you just have to use the right data structures. Quadtree is the obvious one if you have lat/long, or if you have cell history you can observe that e.g. the UK only has ~25,000 base stations, so you can just use them as buckets. It's probably too expensive to run on every ad load, but for targeted surveillance it's easy.

Contact tracing will go away middle of next year with the end of the pandemic. Although I wouldn't rule out its reactivation next time there's a SARS or bird flu breakout.