Apple Watch SE and ECG - Watch OS 7 worked but not OS 8

There is an Apple Watch SE and an Apple Watch 6. There is no Apple Watch SE (Series 6). Apple Watch SE does not include ECG. Apple Watch 6 does. Which watch do you actually own?

I don't know what to tell you. Apple Watch SE does NOT have ECG. See below:

How to use the ECG app

The ECG app can record your heartbeat and rhythm using the electrical heart sensor on Apple Watch Series 4, Series 5, Series 6, or Series 7* and then check the recording for atrial fibrillation (AFib), a form of irregular rhythm.

The ECG app records an electrocardiogram which represents the electrical pulses that make your heart beat. The ECG app checks these pulses to get your heart rate and see if the upper and lower chambers of your heart are in rhythm. If they’re out of rhythm, that could be AFib.

The ECG app is currently available only in certain countries and regions. Learn where the ECG app is available.

*The ECG app is not supported on Apple Watch SE.

Then you'll have to address this with Apple. What I posted above is directly from the Apple Link on using ECG on Apple Watch. Click here --> Take an ECG with the ECG app on Apple Watch - Apple Support

I would think it's more likely, you don't have an Apple Watch SE, but you have an Apple Watch 6. Why you cannot take an ECG on an Apple Watch Series 6 I don't know. Did you move to a country or region where ECG is not yet approved for use on Apple Watch?

How lucky for you! If you remove the watch from your wrist, the model is clearly noted on the back of the watch.

Now, to figure out what happened to ECG? Did you move, or do you live in country where ECG is not supported? Apple WatchOS did not remove ECG. But you could live somewhere, where it is not permitted.