iOS: Before you download iOS 11, you should make sure all the apps you rely on will work with the newest version of the operating system.
With iOS 11 Apple will no longer be offering support for 32-bit apps. Instead, everything that runs on iOS devices will need to have been updated to a 64-bit version. On a super basic level, 64-bit apps are going to run much faster than a 32-bit version and take advantage of the current hardware in the iPhone.
If you want to learn more, Digital Trends does a pretty good job of explaining the difference between the two. This statement sums it up: "64-bit processors are to 32-bit processors what the automobile is to the horse-drawn buggy."
Apple has been supporting 64-bit apps on the iPhone and through the App Store since the launch of the iPhone 5s in 2013. All devices after that have supported 64-bit architecture, and every app launched in the App Store or updated after 2015 was required to come with 64-bit support.
That means that for most apps, the developer likely updated their app a long time ago -- since Apple has been pushing developers to move to 64-bit for several years now. That said, some might have still been dragging their feet (or haven't updated their apps in general since Apple made that call).
iOS 11 is now available for download, which means you can upgrade one of your many iOS devices to get features such as a smarter Siri, a new app dock on the iPad, and improved multitasking support. It also means you'll be downloading the first version of a major software upgrade, which isn't the best idea, especially on launch day. Issues ranging from slow download speeds to frozen devices often plague the first version of new operating systems, and the potential for something to go awry on day one should be enough to make you consider waiting until Apple irons out the kinks.
Apple even put out a last-ditch call for developers to update their apps to 64-bit in June.
To find out if any of your apps will stop working, you can go into the Settings menu of your iPhone, followed by "General", then "About", then "Applications on Your Phone". The next screen will show you a list of all of the apps you currently have installed on your device that haven't been updated for iOS 11.
If any of your favourite apps are on that list, your best plan of action is to just find a replacement. At this point, if a developer hasn't updated their app yet, they probably aren't going to. They should know their app stopped working. If you really can't live without something, you may want to reach out to the developer directly to see when and if they plan on updating.
Still, it can be good to know you're SOL before you update rather than after. Once you have iOS 11 installed, it will be too late.