I don't recall ever seeing any review mentioning that this device would have any sleep apnea tracking or functionality. Here is an excerpt from a review page...
AllGalaxy Watch 3 models come with a built-in pulse oximeter for measuring blood oxygen levels, or SpO2. Manually recording blood oxygen data from the watch is pretty seamless, though I’ve noticed it does take a few seconds longer than my Fenix 6 Pro. It’s also very touchy in regards to movement. For what it’s worth, the recordings on both Samsung and Garmin devices have been pretty spot on, usually hovering around 97%.
Samsung isn’t trying to detect signs of sleep apnea like some of its competitors, so the pulse oximeter only works for on-demand readings. It can’t run at night while you’re sleeping.
I think the issue would be with getting the functionality passed through medical testing. Samsung gained their EU Accreditation for the Blood pressure and ECG montoring and clearly they must conform to strict health standards.
You can't just implement a feature in a watch without having to jump through hoops with the medical testing and standards. If it doesn't quite work then it could cause litigations and legal battles.
And if it mis-reads signs of Apnea, then the NHS would not be thankful with having a host of their Galaxy Watch owners turning up on the doorstep stating that they have Sleep Apnea, when the reality is they are fine, and it is a data misread.
I suspect the SP02 levels are not consistent and reliable enough to be able to give reliably good data during sleep.