I created an account just to say thank you and to provide more information in case anyone lands on this thread in the future.
I have an iPhone 8 and had to return the Series 9 Apple watch, not realizing they were not compatible. Speaking to 3 different Apple support people about what model would be compatible (and also provides the ECG feature) did not help much, as none seemed knowledgeable about this topic. This thread got me on the right track.
I ended up discovering that the Series 8 watch is compatible with the iPhone 8. One thing I suspect, but am not able to confirm, is that it may depend how long the Series 8 has been "sitting on the shelf" so you should try to see the sticker on the back of the box. (I was going to upload a pic of the one I ended up buying that worked but this site doesn't allow iPhone pic format/HEIC--odd for a Apple-based website?).
In my case, the sticker on the box says it's compatible with iPhone 8 and iOS 16 or later AND it was new/never opened so I knew the OS had not been updated. I found it on eBay. I did not consider buying any series 8's that were for sale where that sticker was not shown in the pictures, as I suspect those that may have been more recently manufactured may be compatible only with iPhone Xs or later and iOS 17. There are some online resources that specify that is the case for the Series 8 watch. One of the Apple reps told me that as well. But there were other resources that aligned with what the sticker said on my box. So my theory is it might be both (iPhone 8 or later vs Xs or later), just depending on when the watch was produced.
I ordered the eBay watch and everything is working great. I thought I might be forced to do an update, but no, it's just what I hoped for. I hope this helps someone else. I appreciate that the people that responded to the OP applied more logic to the question vs. just defaulting to some canned Apple compatibility literature. The key was figuring out which watches were not on WatchOS10 and then kind of following the bread crumb trail from there.
Thanks again. I was pretty upset when I realized the series 9 wouldn't work and had all kinds of negative feelings about Apple and their business model but then found a happy solution in the end after stumbling on this post.