The old course (Mastering React) will stay in library since there are many legacy React apps built with class components. Take the old course only if you have to maintain an older React project that was built with class-based components. For new projects, I encourage you to explore our latest React 18 course.
Mosh mentioned in his YouTube channel that this is an ongoing issue and will be resolved soon.
To confirm this you can look into his React Course video (YouTube) and in the comment section someone mentioned this issue where Mosh replied above info.
Look for comment with name: Myndale
If you have already purchased the old react course, just watch all the lectures and make notes of the concepts because you will have hard time to execute the code as the versions are outdated for node, react and other libraries used in the project.
Once you are through that course, new course will be a walk in a park for you.