If this gets any sort of public traction it'll be built in to shop doorways, public transport, police cars, and street lights within a couple of months.
Yeah, in a new york minute. This is already built into iOS in their newer Find My Phone. The carriers also know who and where we are just need to add in covid testing data. The problem is that even if it is decentralized and OSS, then the average user can't/won't install it. If it is simple and easy (centralized), then it becomes a honey pot for the government.
You can't, but apparently Find My Phone uses Bluetooth to find nearby phones and thats the same technology that Bluetooth contact tracing for coronavirus would use.