Spring 5.0 embraced the ReactiveStreams specification and introduced a new Reactive Stack as an alternative to the traditional Servlet stack. And, it also brought a new functional programming model to developers, but it is only supported in the Reactive Stack.
The good news is that, in the coming 5.2, the functional-like APIs are being ported back to the Servlet stack. For those developers who are stuck on Servlet stack and want to experience the new programming model, it is absolutely startling news.