Thank you for the answer, @ZenLeviathan!
I have also seen the new platform, but unfortunately the content is missing (also on Youtube). As I have read here, apparently the courses that were considered outdated were not migrated to the new platform. I am not sure how that was decided but in this case I feel it was the wrong decision to not offer the course anymore.
The other course your are mentioning probably is a good resource but does not address the technical aspects of creating a plugin, but only the marketing aspects in case you want to publish it on the marketplace.
So I guess there is no chance that Epic Games themselves publish the course anywhere again?