Is this used outside the Apple eco-system much? I admit I haven't looked at Swift in 2 years or more.
I find the omission of features like namespaces really strange. I realize that namespaces don't jive with how Obj-C and Obj-C frameworks see the world and that Swift and Obj-C need to play together, but dealing with global namespace pollution and collisions in 2018 just seems...sad? They're hobbling the language in various ways due to NS*/Cocoa framework stuff.
Another weird omission is lack of any kind of concurrency story. Right now, this just seems like Apple's "me-too" version of Java/C#, but what does it actually bring to the table that would make it a worth-while investment if I'm not an iOS developer?
Grand Central Dispatch (GCD or libdispatch) is generally the concurrency story on Darwin platforms. There has been an ongoing effort to port it to Linux. Not sure how complete that is but I linked the Github below. Swift on Linux still has a lot of rough edges in general.