This, right?

https://github.com/swiftlang/swift-foundation

Yeah I jumped into swift on Linux a while back having mostly used it on apple platforms and I couldn’t even tell anything was different. A few years ago I would’ve had to struggle with SwiftNIO but not nowadays. URLSession, Codable, etc. all there on Linux (not sure about Combine but Combine is stupid in the Swift 6 world IMO. Swift concurrency is better in almost every way).

Swift on Linux (except NixOS) is actually very good nowadays. There’s even a libadwaita library that feels a LOT like writing SwiftUI.

Feels like a lot of folks were turned off early on, found something else, and never bothered to try again (which is fair).

I also have a dim view of Combine and Swift's shitty observation regime, but what does its concurrency have to do with it?

Swift Concurrency as a feature set includes async/await and async for, which solve a large part of Combine's same problem with better safety and less setup/teardown. These days Combine is still useful, specifically for multiple observers and several cases of adapting to older event publishing sources.

Combine is for setting up observers to changes, whereas Concurrency is for async operations. I don't see the relationship.

Foundation wasn't made to be part of the Swift project until recently. 25 years ago it was the "foundation" of Cocoa, the Mac OS X API derived from NEXTSTEP. It was an Apple platform thing explicitly—now it is remade in Swift and is part of the Swift project.