I don't know, if they apply semver but just go from major version to major version, that's no change in policy. React 15 -> React 16 makes no BC guarantee.
The way the announcement was presented at React.js Conf this year suggests that the version shift also represents a change in stability. Personally I think that from now on React will treat major releases more like Node does -- but of course many people would still consider that pace anxiety-inducing.
