Searching for HTML Help Workshop on https://www.microsoft.com/en-us/search?q=HTML+Help+Workshop gives back a link list and the first link is leading to Microsoft HTML Help Downloads.
So, there is a misleading note (running one into a loop) on top back to the download Center "All HTML Help downloads are now available from the Microsoft.com Download Center.", where HTMLHelp.exe download and zipped docs cannot be found.
But, the really problem are broken links for the HTML Help Workshop including the CHM compiler. Generating CHM's as an only working Windows application help is not possible for developers when e.g. a new environment must be setup.
The compiler for CHM files is still needed after more than 20 years, because no replacement for CHM help files has been provided by Microsoft.
The files should be made available in the Download Center.
Where can the HTML Help Workshop and documentation be downloaded?
Ulrich [Microsoft Help MVP 2007-2009]