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. There are new lifecycle methods to replace them. The main issue is that the deprecated lifecycle methods encourage coding in ways that doesn't work well with the new async React core.

For more on these lifecycle methods, visit this page: https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html.