On macOS, DispatchSpecificKey is defined like this:

final public class DispatchSpecificKey<T> { public init() } extension DispatchSpecificKey : Sendable where T : Sendable { } 

on Linux the conditional conformance to Sendable is missing.

https://github.com/apple/swift-corelibs-libdispatch/blob/e85f6a0d5c9ea1f32f5013c3fa34e4fc146cd0eb/src/swift/Queue.swift#L18

The end result is that code with strict concurrency checking that does compile under macOS with Swift 6 does not compile under Linux.

See the associated post here: https://forums.swift.org/t/dispatchqueue-setspecific-swift-6-concurrency-linux/74653