So, rolling it around in my head, I'm realizing that I can't equate iOS apps with macOS apps: in macOS, an old version of an app can run in an old version of the OS because it's "self-contained," but the iOS apps with which I've run out of luck are kinda like a cross between an app and a website, and an old version of the "app" can't connect with the new version of the "website."
Does that make sense?
Sandboxing does not mean an app cannot use or be dependent on API (Application Program Interface) functions provided by iOS/iPadOS/macOS, it means the apps have no mechanism for accessing data or functions
in other apps. In fact, those APIs are essential for the apps to function. I don't know about the Chase or Discover apps but since they access the internet and are likely dependent on JavaScript and HTML code it is just about a given that they both use APIs provided by the copy of
Webkit in Safari. athe last time I checked, all browsers, Mail and similar internet apps on iOS, including Chrome, Brave, and even Firefox, use Safari's Webkit instead of their browser engine they use in macOS. (
BULLETIN: Mozilla is reportedly working on a non-webkit version for iOS)
And yes, it does make sense that an old version of an app or iOS cannot properly interact with the latest version of a website. There is no backward compatibility requirement in the W3C standards, governing HTML and JavaScript, so a site using a newer version of HTML or JavaScript may not work with an older browser engine version that does not know how to interpret the new or revised site code.