The Legacy Responsive Web Design is the old curriculum. It was replaced by the new one you just finished in 2022. The new one is project-based, meaning it teaches coding concepts through direct, hands-on project-building step by step, whereas the old used to teach each concept individually. This was part of a major improvement plan that will eventually be applied to most of the curriculum, I believe. The idea is that project-based learning is an improvement on the old way of individual concept study; however, you can use the Legacy curriculum too if you like. It might be helpful if you need to reinforce some concepts you’re struggling with. Other than that, it’s not necessary.
Here’s a post with the founder announcing the beginning of these changes in 2019:
Happy coding!