I would expect the opposite mechanism that what your title says - empirical evidence shows ECG is disabled everywhere by default and only enabled where cleared. This is also observed for COVID-19 location tracking. It’s baked in to iOS but enabled selectively by Apple in specific regions.
For example, warchOS 7.3 expands ECG in Japan, Mayotte, Philippines, and Thailand. Irregular heart rhythm notifications are now available in Japan, Mayotte, Philippines, Taiwan, and Thailand.
Behind the scenes, I expect this activation happens once which writes a configuration profile so the OS can read the value from the Secure Enclave which makes it tamper resistant and survive iOS restores or other regional settings.
For most iOS based restrictions, Apple pins the country for the OS specific features to the sales process. Each serialized product gets sold in a specific location which then lights up the features intended for each region.
When Apple chooses to add the feature to a new location, it would push a software update to interpret the stored values differently and not that it would change the value of the “sold-in” region within the Secure Enclave.
If this is the mechanism, a theoretical jailbreak could easily enable ECG by patching the routine that checks for a valid region even if it can’t change the record of sale stored on the device.
I would open a direct Apple support request for them to explain to you why you can’t change your region if that’s something you can do or have done, but I wouldn’t expect them to re-purpose a device sold in another region for legal and possibly also significant logistical reasons.