Correction: rewrite of PARTS of Foundation
There already was an open-source project to rewrite ALL of foundation, but it had stalled on the shores of having to re-implement everything:
https://github.com/apple/swift-corelibs-foundation
The news is actually that Apple is now instead trying to define the bits/parts to support via Swift on all platforms (the original API's will always be supported on Darwin).
The announcement:
https://www.swift.org/blog/future-of-foundation/
The discussion, with hairy details about which bits, esp. for async:
https://forums.swift.org/t/what-s-next-for-foundation/61939/103
The plan is to divide up Foundation into more- and less-essential parts, to get more-essential parts locked down so people can rely on them.
What's Foundation? Swift's most-core library is the stdlib, tightly coupled to the compiler/language version, providing things like arrays, dictionary, etc., and available wherever Swift is. Beyond that, Foundation is the library with core API's for common features, e.g., for dates and concurrency. Stdlib is fully cross-platform and Swift-specific, but Foundation is a beast with API's dating to NExT with support for 20 years of API's.
Microsoft famously arrived at porridge for an operating system by maintaining backwards compatibility. Apple has cracked open Swift by developing in the open, but it's still Apple-funded and Apple-driven. Library and some integration support for other platforms has always had to come from community (notably compnerd's heroic effort to make-things-work on windows, and a revolving cast wanting Linux support for their server API's).
But there's no good reason to impose the whole Foundation history on other platforms. And there may be a movement inside Apple to migrate internal code to newer async API's, designed after the recent Apple-silicon generation.
For developers with server experience and some free time in a tech lull, it could be a good opportunity to help rebuild a new, er, foundation for computing on all devices, that's native but type- and memory-safe. The community is large and mature, but there is plenty of room for others.