An indication of this was found in Samsung Health in the beta version of the One UI 4.5 shell for the current generation smartwatch. The Watch 4 line does not have any temperature sensors, so the update is clearly for the next generation.
As you can see, there is a setting to monitor the temperature during sleep, and this suggests that the watch will be able to determine body temperature.
True, only the older Watch 5 Pro may be able to do this, but that’s just speculation. There is already a watch on the market with such a sensor, but so far it is very rare. In addition, measurements are often very inaccurate.