Why has itself and the evidence it ever existed been removed?
Only a half year ago it was promoted on channel9 as Something that should be in everyones pack -- today the Microsoft Archive is "retired" and totally keeps quiet that the API code pack ever existed.
Seems like someone also posted it on CodePlex.
I can't answer the question of why, nor do I wish to speculate (maybe someone else closer to the issue can tell us what happened), but thankfully it's not completely gone. It looks like the binaries have been reuploaded to NuGet by other developers, although note that none of the packages are official ones from Microsoft.
The following packages were uploaded by NuGet user aybe:
As usual, to install them:
A zip file containing the same files as the original executable self-extractor (i.e. the source, binaries and docs) is located here: https://github.com/jamie-pate/KeepSync/blob/master/contrib/Windows%20API%20Code%20Pack%201.1.zip.
Here's link (or this link) to original Windows API CodePack 1.1.
The original links are dead, but an Microsoft employee has uploaded the v1.1 original codepack to nuget. Search for winapicp at nuget, or use this link: Windows API Code Pack for Microsoft .NET Framework
Someone has uploaded it to GitHub.