I don’t feel there are many huge plot points that you need to remember. For the most part, the last season largley re-united most of the characters in Hawkins.
From what I remember of the last season:
They did the absurd resuce of Hopper from Russia. And did all the back story of the main villain. I kind of forget what most of the kids were up to, since it seemed mostly irrelevant. I think Max was still injured from the main villain Veccna coming after her.