With iOS 11, Apple is making a big underlying change to iOS. It is completely dropping 32-bit support from the OS. This means that iOS 11 will only work with iPhones and iPads that are powered by a 64-bit A-series chipset. To go along with it, the company is also dropping support for 32-bit apps from the iOS App Store.
This means that going forward, developers will not be able to upload 32-bit apps on the App Store. Additionally, for existing 32-bit apps on the App Store, Apple is encouraging developers to update them with 64-bit support. This should not be surprising since Apple started showing a pop up in iOS 10.3 whenever a user opened a 32-bit app saying that it needs to be updated with 64-bit support. Without 32-bit support, iOS 11 should be a fair bit lighter in terms of size and resources which should help improve performance on older devices.
As for devices, iOS 11 drops support for iPhones and iPads powered by 32-bit A-series chipsets. This includes the iPhone 5, iPhone 5c, iPad 3 and 4. All other newer iOS devices will be supported by iOS 11.
Apple will likely also drop support for 32-bit instruction set from its A-series chipset entirely with the launch of the iPhone 8 later this year. This move will allow the company to free up die space on the chipset which can be used to offer a bigger GPU for better performance or introduce other blocks to improve certain performance aspects of the OS.
So, if you are a developer who still has legacy apps on the App Store, do update them before iOS 11 drops later this year.