I've been arguing with and I think finally managed to defeat the Oreo update on my S8. It's now stuck on Android 7 and all the nag messages to update it have gone. This required disabling several packages on the phone and a particular sequence is needed to achieve that.
Whether it is of interest or not: I did note the phone seemed much slower, even though I didn't upgrade it. That seemed to coincide with when the phone detected and began nagging about the upgrade.
Eventually the phone became more and more persistent about the upgrade and the battery life seemed to worsen. However "seemed to" is hardly proof.
In succeeding in resetting the phone and disabling Software Update completely I went through a process of resetting it multiple times. Each time I'd note that the moment the software update nag messages became prevalent, the performance would drop, as did the battery life. With almost nothing installed on it.
The suspicion that I have, and it is only that, is that the degradation begins when the update nag messages become persistent - move to level 2, if you like - even before the Oreo update. Based on witnessing the same thing three times. The key is to stop the phone from even detecting that an update is available.
Now I've overcome all that and it's back the way it was out-of-the-box with plenty of apps installed, it's like greased lightning again and the battery life is much more stable.
Incidentally, and confirmed by Samsung - the phone ignores the Developer Option (Auto Update System) and the Software Update option (Automatically Download Updates). It's a fault out-of-the-box, some might say, by design.
Those settings do literally nothing and will not prevent the phone from downloading the update and nagging about it, a more hardcore approach is necessary.