i still have a series 3, the battery still last me a day and a bit without any workout. The darn thing won’t die. Let’s see if the series 10 has anything worth upgrading to.
I noticed it on my series 7. Before I got at least a bit more than 24 hours of normal use with the screen always on. Now it is getting about 4 or 5 hours less.
I think when Apple announces battery life for watches, the measurement is for the current watch on the current software. Over time, yes your battery holds less charge, but also, the software gets new features that the newer hardware handles just fine, but the older hardware starts to struggle with. Apple doesn't provide data for newer software on older devices. There's no spec sheet from Apple saying, "if you upgrade your series 5/6 to WatchOS 9, you should expect this new, lower battery life."
But we all know newer, better features are going to use more compute - that's just how technology works. So you either 1) cut off older devices from the newer features, which users get grumpy about, 2) provide the new features but don't say anything about the impact or 3) provide new features but say how it impacts older devices and maybe provide toggles and system controls to enable/disable. I don't think #3 is really in Apple's DNA - that's more of a Microsoft approach.
The chip in the Series 6 is essentially identical to the one in the Series 9 and all in between. Your explanation works for a lot of cases, but not this one.
I had a similar issue on watchOS 9 and apple after many attempts would not help me revert the update which seems to have caused the issue. I ended up having to pay for Apple care and get a replacement watch, which I refuse to let update. Pretty annoying. I definitely won’t be allowing my current one to update to version 10 now. It’s sad that the bar is this low, but at least Apple doesn’t force you to update in this case (at least so far)