Since next Mobx update might be major due to native decorators support, it makes sense to revisit the API and remove deprecated features. Removing deprecated features gives less code to support for maintainers. For example here while mocking console.log I had to support both observer and useObserver usage:

Here is the list of deprecated API I've found:

• useObserver. The goal is to only remove it from public API since it's still being used internally:

  mobx/packages/mobx-react-lite/src/ObserverComponent.ts

  Line 13 in 0143275

  return useObserver(component)

• useLocalStore. It's deprecated in favor of useLocalObservable.
• useAsObservableSource
• Batching utilities like batchingForReactDom.js, batchingForReactNative.js, batchingOptOut.js:
  

  mobx/packages/mobx-react-lite/batchingForReactDom.js

  Line 2 in 0143275

  console.warn("[mobx-react-lite] importing batchingForReactDom is no longer needed")

• isObserverBatched since it's always returns true:
  

  mobx/packages/mobx-react-lite/src/utils/observerBatching.ts

  Line 21 in 0143275

  console.warn("[MobX] Deprecated")

• options argument in observer:

  mobx/packages/mobx-react-lite/src/observer.ts

  Line 65 in 0143275

  // TODO remove in next major

• Drop flow type checker support, types and tests since flow is no longer supported: mobx-undecorate problem with flow js nullable types #2792 (comment)

Feel free to add missing API 👍 Once we agreed, I'll start working on PR.