Providing apps for older devices

>I downloaded Xcode 11.4.1 and quickly found that projects created are not backward compatible with the older devices.

The connection between Xcode and 'compatable with the older devices' is through the iOS version of the app and on those older devices. Older devices cannot run newer versions of iOS. The most current Xcode can write code that targets down to iOS8.0. That version of iOS will not run on certain older devices (e.g. iPhone 4) and so, no, you can't write a new app or update an app and have it run on those older devices that can't run iOS8.0* or later. But if you select iOS8.0 for your Xcode project, and if you do not use any of the newer APIs that are not compatable with iOS8.0, then your app can be approved, downloaded and run on many 'older devices' - but not all.

*And the same applies to iOS9, 10 and 11 with each iOS losing one or another additional older devices.

I am using an old mac too, if i want to test historic programms which cannot be virtualized on my newer mac. The older mac than supports the older devices over direct connect via usb cable.
But as others say, there is a lifetime cycle for both - app development and devices. The lifetime cycle by apple is much longer, than by any other devices or systems. Using a device as long as possible is a strategy, also supported by apple. But sometimes it could be helpful to rethink about the own lifetime cycle support, because with new technology new capabilitys are available. Accessability for example is a topic, which is much better included in the modern development environment, than in the older versions.

Thank you for your help!!! Be safe!