https://github.com/apple/swift-evolution/blob/master/proposals/0204-add-last-methods.md で

The name changes to index(of:) and index(where:) are easily migratable, and will be deprecated in Swift 4.2 before being removed in Swift 5.

って書かれてるけど、Swift 4.2ではindex(of:)とindex(where:)のどちらもdeprecatedになってなくて、5.0でdeprecatedなのね。 https://github.com/apple/swift/commit/e8b6091755fc87241341e82845a29043361e8e9f

apple/swift-evolution

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - apple/swift-evolution

[stdlib] Deprecate Collection.index(of:) and index(where:) (#19071...

This should have been done for Swift 4.2 according to SE-0204, but better later than never. Deprecating these methods as of Swift 5.0. Also modfying the tests to check for the deprecation messa...