Is it on the arxiv? If not, please put it there.
As its author noted, the paper has done fine ciation- and impact-wise.
Your link is the website I put up for non-experts when I announced the issue.
If the course website was even on the open web to begin with. If they're in some university content management system (CMS), chances are that access is limited to students and teachers of that university and the CMS gets "cleaned" regularly by removing old and "unused" content. Let alone what will happen when the CMS is replaced by another after a couple of years.
https://wiki.archiveteam.org/index.php/University_Web_Hostin...