Whoa this is a big one! 6.4.0 brings all the data loading and mutation APIs over from Remix. Here's a quick high level overview, but it's recommended you go check out the docs, especially the feature overview and the tutorial.

New react-router APIs

Create your router with createMemoryRouter
Render your router with <RouterProvider>
Load data with a Route loader and mutate with a Route action
Handle errors with Route errorElement
Defer non-critical data with defer and Await

New react-router-dom APIs

Create your router with createBrowserRouter/createHashRouter
Submit data with the new <Form> component
Perform in-page data loads and mutations with useFetcher()
Defer non-critical data with defer and Await
Manage scroll position with <ScrollRestoration>
Perform path-relative navigations with <Link relative="path"> (#9160)

Patch Changes

Path resolution is now trailing slash agnostic (#8861)
useLocation returns the scoped location inside a <Routes location> component (#9094)
Respect the <Link replace> prop if it is defined (#8779)

Full Changelog: v6.3.0...v6.4.0