Is it not possible because the iPhone 8 cannot actually run 15.8.1, or because Apple wants to force an upgrade to iOS 16 and is therefore deliberately not singing 15.8.1 for the iPhone 8?
I suspect the answer is the latter because my iPhone 6s Plus has the update for iOS 15.8.1 ready for me to install. And if the 6s Plus can run 15.8.1…
Go with the latest you can find for your device, iOS 16 has been around for a long time with some nice improvements and is still updated fairly regularly (no feature updates, just bug and security fixes). There are updates that you would want to fix some severe security flaws.
Is it not possible because the iPhone 8 cannot actually run 15.8.1, or because Apple wants to force an upgrade to iOS 16 and is therefore deliberately not singing 15.8.1 for the iPhone 8?
I suspect the answer is the latter because my iPhone 6s Plus has the update for iOS 15.8.1 ready for me to install. And if the 6s Plus can run 15.8.1…
It’s not possible because Apple doesn’t allow users to install the latest point version of an older iOS version if the device supports a more recent major version. Otherwise, downgrading would be possible (unless it’s only via OTA, and this is still not the case, barring some time after a major version is originally released, timeframe in which they allow this... temporarily, of course).