What are the devices and iOS versions? We do not have a document outlining this but generally, if the device is old and/or it is not receiving updates from the OS manufacture, we can't really provide support for outdated technology. It really would be in an "as-is" use.
Of course, if you're using technology, like CloudFlare, to transpose your images to WEBP and the device doesn't support WEBP, that is outside our hands.