Not sure why this is on the front-page. This is just a glue library between two other libraries and the update doesn't seem to be a real major update.
"Version 5.0 maintains API compatibility with v4.x but due to major internal changes and potential behavior differences across nearly all API surfaces, semver dictates a major version bump. Also, it's good marketing!"
React and Redux are both popular and widely used, and this is the official library that ties the two together. Also, it's absolutely a major update. The API is almost identical (with one new function), but the internals were completely rewritten, with potentially different (but better) behavior in a few edge cases. It also offers improved performance in most cases.
"Version 5.0 maintains API compatibility with v4.x but due to major internal changes and potential behavior differences across nearly all API surfaces, semver dictates a major version bump. Also, it's good marketing!"
Anyway, the marketing part worked. :)