There are some badges I see in people's profiles - such as Algorithms I, II, Data Structures I, II.
However I don't see study plans for them. Is there another way to get them? Or those are gone forever?

Also it is not very intuitive to understand how to get badges, except for the monthly and yearly. Is there an official post by LeetCode that describes what badges are present and how to get them?

Suggestion: It would be helpful to have a pinned Post by LeetCode or when you open a badge to tell you a description on how it is acquired.