The Boundless PostGIS workshop is an excellent introduction to PostGIS. It can be argued that it is dated, but I think the fundamentals are still very valid. However maintaining a free course is quite an overhead for any company. So I don’t blame them from taking it down, and no longer hosted it on the Boundless website.
However it was released under a creative commons license and the source is easily available on GitHub, and can be easily built.
You can find a hosted version on: GISforThought.com/projects/postgis_tutorial/
The data can be found in my previous post: GISforThought.com/postgis-tutorial-data/