We make every effort to test and provide support for customer on the minimum required versions of our apps. And whilst we do not foresee any changes to the minimum iOS/OS requirements in the next few years, technology will require us all to keep pace. There may be a time in future when we have to revise the current requirements; this would only be after all due consideration and with lots of lead time for our customers.
App performance can be affected by ageing or older hardware, even if we support the newest version of the OS or iOS. This is because operating-system updates often leave older hardware behind - this happens gradually, but eventually, for all devices.
For example, let's say your worker's iPhone 6 from 2015 receives the new iOS update - even with the newest operating system, the hardware might still lack the processing speed to run an app that requires more efficient architecture. In this case, the iPhone 6 might struggle to upload a form with many images. The number of apps open and the resources available could just be too much for it to handle now.
Older devices have been around longer and take a beating. Unless you are clearing them regularly, most (if not all) of the available storage, will get used up.
Signs of age can include frozen screens, slow-loading apps or apps that just don't load at all.
We would all like to keep our old hardware a bit longer, defer the expense or hang on to it for the features we know and love - but the cost of replacing older devices is one that we all will experience sooner or later. Keep in mind, the average lifespan of mobile devices is 2.5 years.