Not likely to ever be included but always a possibility, how about that for hedging?! But it would serve no purpose. Getting a relevant and useful body temp can’t be done from your wrist. It’s variable enough when using those forehead scanners to check for elevated temps or fevers but at least across your forehead you do have a lot of superficial capillaries that can provide a reasonably accurate reading.
The thermometers that don’t actually touch the skin, the infrared ones, are even more variable and highly dependent on the ambient temp where you are. The Apple Watch will scan a range of temps while asleep and can track that variability to give ovulation guidance. It doesn’t need nor can it do an exact core body temp.