April 2021
I started out updating parts of the bootstrapFiles manually whenever I ran into a new problem that required newer tools or libraries. This became harder and harder to do because changes to one part of the bootstrapFiles required changes to other parts and it got messy. So the first thing I focused on was generating the bootstrapFiles properly using make-bootstrap-tools.nix. With some changes so I can generate new bootstrapFiles while passing in new bootstrapFiles.
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I’ve also started updating the LLVM version used for Darwin, both the stdenv and the default for packages. The PR can be found here. This is long overdue and has held back the default LLVM version even on Linux (though voices have been raised to change this in 21.05). I’m hoping this solves a build issue involving Swift CoreFoundation and macOS version 10.13.
Some exciting news is that @abathur’s work on improving the installer has been merged 
They’re currently looking for someone to pick up the project of completing full uninstall/reinstall support as they’re short on time. (I believe this wouldn’t be specific to the macOS installer.)