For the most part, Apple prefers to maintain backward compatibility with older applications.
So, when they finally cut out something, it will be because maintaining it has become too expensive, or because maintaining it will add some new function is difficult or impossible. (And this reasoning may include reducing obsolete code due to the system becoming too large to fit into the device.)
The best reason for not using legacy methods is the application approval process. Apple may require new submissions to avoid legacy code long before that code is actually cut.
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