"String/NSString API reconciliation is something that should happen soon" -- @Michael_Ilseman
A good place to start is to look at NSStringAPI.swift in the Foundation overlay. At the top are the APIs, and at the bottom there are many unavailable/deprecated declarations for things that have idiomatic alternatives in the standard library.
However, many of those APIs deal with localization (currently considered out-of-scope of the standard library) and NSString's Encodings (while the standard library only really knows about Unicode encodings). There may be multiple rounds of yak-reconciling here.
hot take: NSString methods should not be shown alongside native String methods in the Swift documentation
This was discussed previously.