Really, the author simply took and removed links to direct download? I don't understand this. Maybe MS paid him for advertising NuGet?
Disclaimer: I'm the owner of the project Html Agility Pack
CodePlex will be shutting down, so keeping a version up to date on it doesn't make sense. In fact, it's still currently possible to download the v1.4.6 (not the latest) on CodePlex: https://htmlagilitypack.codeplex.com/
HtmlAgilityPack (HAP) Binaries are now available for download only from NuGet.
This has been done as a standard as well as ease of maintenance.
If you want to download the sourcecode and compile, you still can do it by downloading the source from GitHub: https://github.com/zzzprojects/html-agility-pack