Seems odd to sort and present to users top courses based merely on the fact that people from years back clicked on it, checked out the course, and didn’t bother to go through the steps of deleting the course from their list.
Memrise should offer additional sorting by activity (last 7 days, last 30 days, all time). Activity is account answering at least 1 item.
Also, they should offer a method to sort courses by “total users completed”. Only courses with at least 50 items are shown. “Complete” just means a user achieved a minimum of 500 points per item. So learning a 50 item course needs 25,000 points to get counted as completed for sorting purposes while a 200 item course needs 100,000 points. This is arbitrary, but it’s something users can see for themselves if they sorted by top all time users of course.