> This is only an issue for broken symlinks, right? (More precisely, those that cannot be resolved, which may include very long paths on some machines.) 

Unfortunately, no. In the original post above, you can see temp/bar points to C:\Users\jaraco\temp\foo, which exists. Yet os.readlink returns \\?\C:\Users\jaraco\temp\foo. That's the root cause of the issue with copytree.

> what do you need to be able to test against prerelease? It would be nice to find these impacts during alpha/beta rather than three bug fix releases later.

I need a mechanically-reproducible technique to test dozens of projects against pre-releases on Windows. Historically, I've had Travis as my prime CI (non-existent Windows support) and AppVeyor for select projects that demanded some Windows testing. I've recently started migrating to Azure Pipelines, which has nice multi-platform support and that's why I've started catching these issues. Presumably, I'll be able to add a pre-release of Python 3.9 to the pipelines config at jaraco/skeleton - that will ensure that all projects I maintain going forward will get tested on the pre-release.