Guilherme Rambo, on 9to5Mac:

This limitation is not enforced by hardware, and if you buy an Apple Watch Series 4 in any country, it includes the new sensors. Looking at code within iOS related to the ECG feature, we’ve been able to confirm that its limitation to the US will be based on the software region of the user’s devices (iPhone and Apple Watch).

I wish Apple just required users, in areas of the world without regulatory approval, to understand that the ECG function is for their own personal use and that they shouldn’t bother sharing the information with doctors — a clearly worded pop-up or instructional screen should be sufficient. I don’t plan on switching my region to anything else because this is a hassle in a number of areas (which I don’t recall now but I remember doing it once and a number of things frustrated me).