I never heard such a rule. Anyway it's appalling that a release that basically introduced nothing (especially on the iPhone 6 that can't do AR or live pictures) screw up performances. Basically the new iOS is a step backwards: no features whatsoever and worse performance and stability.
Do you actually, spend time outside of MR when discussing Apple stuff? I've never heard of that here, but on Quora and one of my tech groups, it's been brought up. Getting a 3rd, and perhaps a 2nd major iOS update runs the risk of your phone taking a performance hit, although there have been folks who chimed in that wasn't the case, or that it still runs "fast enough".
I can leave my iPad off the charger and it will last like a month without dying, sounds like something is wrong with both of yours. And possibly an electrical wiring problem if they both stop charging for no reason.
Two different devices, three different cables and chargers and NO problems at all running last iOS 10 release and all these problems immediately after upgrading to iOS11 and persisting a week later ?
May possibly be fixed already quite a few betas have come out since, will monitor going forward after today's beta. Although I usually had apple maps and pandora open when the crashes occurred it wasn't always the case. But I have a feeling the issue may have been caused by pandora app crashing.