The article title calls them add-ons, but have are modified/patched versions.

jhancock · on Sept 25, 2008

The PuTTY code base is not structured for "add-ons". You can only take the source and modify and release something new.

I can say from my experience developing http://shellshadow.com that the PuTTY source is readable, lightwieght and well structured. Many thanks to Simon Tatham for such a simple and long-lived tool!!!