At the moment, accessing a local calypso instance dumps a series of ReactDOM.render deprecation warnings:

While it is not causing any user facing issue, it creates noises to developers that would be the best to eliminate. It might be as simple as updating this line: https://github.com/Automattic/wp-calypso/blob/trunk/client/boot/common.js#L406 , but it might also be more involved than that. When in doubt, we should check in with @Automattic/team-calypso .