React 16 had to revamp some of the lifecycle methods that are available to class components. Some lifecycle methods are deprecated and will eventually be removed because they will be problematic for future async rendering functionality in React. For example, a common way to initialize state in a React component is to use the componentWillMount() lifecycle method. Once this method is removed from React, you can just set the initial state directly as an instance value.
For more information on these lifecycle methods, visit https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html.
