I have found a copy of the installation file (for version 1.3, that is) from Microsoft's site using Internet Archive Wayback Machine, from September 2020:

http://web.archive.org/web/20200918004813/https://download.microsoft.com/download/0/A/9/0A939EF6-E31C-430F-A3DF-DFAE7960D564/htmlhelp.exe

The download takes a while (between 200 - 300 KiB/s) for a 3.3 MiB file, but it looks complete and genuine.

I have checked the included readme and license files (you can just open the installert with 7z or any other archive program, that can handle CAB format), but it's not stating that you're not allowed to redistribute the file (so for example you could copy it to your webserver and always retrieve it from there for your build script).

I have checked the included readme and license files (you can just open the installert with 7z or any other archive program, that can handle CAB format), but it's not stating that you're not allowed to redistribute the file (so for example you could copy it to your webserver and always retrieve it from there for your build script).

No, this is certainly not redistributeable. license.txt explicitly lists the parts which can be redistributed, anything else is not allowed.

However, using archive.org should be a viable solution.

Fixed in r14224.