Apple

You might be able to create a very convoluted way by scheduling a Shortcut every few minutes that pulls the latest SpO2 records from Health and if they’re too low, creates an alarm in a minute to wake you up.

But be aware that sleeping on your arm with the watch will also make the readings drop. Also, the Apple Watch only takes a measure every 30 minutes. So it’s probably difficult to catch an apnoea event.