Well, that is because I thought this may be a general problem. Because since I searched for threads about that topic through the forum, before I created my own, I found out that sporadically some others had posted about several completely different courses disappearing as well.
The courses that disappeared for me were for example:
Learn HTML (which had the new UI when I started it, just like the following)
Its not a huge deal for me to repeat the courses. But as for the fact that I thought this would be a general problem, I thought it might have been for others with other courses with higher estimated amount of time to be completed or something, therefore the thread.