A lot of companies out there still use the class-based syntax. While we mostly teach the simpler function-based syntax, which is probably more popular over-all at this point, we also have some lessons on the class-based syntax as well.
In terms of learning class-based React, a couple lessons to look at:
Frontend 3.3 - Patterns (the lesson specifically on this topic, comparing / contrasting the different ways to make components)
Career 1.2 - Recursion (feel free to skip the stuff on recursion, but look at the topic of "JS quirks" for "this" context issues that are relevant to class-based React, and if you want practice with async in general, look at the other JS lecture content in this lesson)
More possibly useful lessons to brush up on React:
Frontend 3.1-React - Intro lesson, good if you want to brush-up JS, Node.js, and how React projects are built
The next React lessons (3.2-4.4) are all useful, covering different topics in React. For example, if you want more practice with passing info around complex React apps, then check 4.2, or if you want more practice with interfacing with APIs, then 4.4
(Updated in November 2022)