Can’t speak for Android. But...
As an iOS App developer, I can testify that it is actually quite difficult to release Apps , or App updates, that run on significantly older versions of iOS.
Apple from time to time set rules, dictating that from specific dates, all new Apps must be built with a specific version, or later, of the SDK. Each version of the SDK is, in turn, capable of building for a range of iOS versions. You can go back maybe two or three years, it varies, but it’s not infinite.
Devs can set a flag to say that, when a new App version is incompatible with an older iOS, but an older version of the App already exists, then users of the old iOS will still be able to install the old version of the App. That keeps 99% of users happy, but does mean that updates for older versions tend not to happen.