I heard that it was a COVID feature. I guess people thought you would be scanning your friend's foreheads before socializing.
That sensor would be much better if it was integrated with the camera and LIDAR, so that you could "paint" a scene with thermal data by moving the phone around.
I just use the stock Pixel Thermometer app (actually not even sure if it's stock or you have to install it from Play Store). Some developers online have complained that there is no public API for it, which may explain lack of other apps. I have never used it for its health purpose. I only use it for objects like walls and cooking pans.
That's the only plausible scenario, but it also came out in 2023 and people stopped caring at least a year before. And it persisted for a few years so either someone thought they could salvage something from it, or no one had a better idea what to put there.
That probably means the board was being designed a year or two before that... so quite possibly they had put it in the design when it still seemed relevant and the cost/hassle/delay of a new revision of the board design and manufacturing changes made it easier to just leave it there and ignore.
That sensor would be much better if it was integrated with the camera and LIDAR, so that you could "paint" a scene with thermal data by moving the phone around.