As stated earlier, you are encouraged to use RxJava 3.0 if you can. This version will continue to grow and receive new features, while RxJava 1.x has not been developed further since March 21, 2018, and 2.x will be maintained for bug fixes only until February 28, 2021. However, there are other considerations that may lead you to use RxJava 1.x or 2.x.
If you inherit a project that is already using RxJava 1.x or 2.x, you will likely continue using it until it becomes feasible to migrate to RxJava 3.0. You can also check out David Karnok's RxJava2Interop project (https://github.com/akarnokd/RxJava2Interop), which converts Rx types from RxJava 1.x to RxJava 2.x and vice versa. After you finish this book, you may consider using this library to leverage RxJava 2.x even if you have the RxJava 1.x legacy code.
Migration to RxJava...
