I've been checking every day since the release date. I thought Apple was throttling updates to avoid download bottlenecks. However, I just noticed after getting the same "14.8 - Your iPhone is up-to-date" message, the iOS 15 option shows at the bottom of the screen. I think the slow adoption rate is due to the change in semantics: updates to new versions of the same iOS are pushed, versus upgrades to the new iOS are user-requested.