For e.g., with the release of Swift 2.2 (included in Xcode 7.3), the curried function declaration syntax was deprecated. You can read the Removing currying func declaration syntax here. The premise being the declaration syntax:
is of limited usefulness and creates a lot of language and implementation complexity
You can also get a sense of why a feature is introduced and sometimes, background information into how it is useful, and might change in the future. e.g. Swift Language Version Build Configuration talks about the
#if swift(>=2.2) syntax for checking Swift language version.
Swift is still a very young language and tracking these changes can help enormously in being able to utilize it efficiently as it changes.