Likely to cause new Flow errors:

• $Keys now produces a more precise type, which may find errors where incompatible strings were passed to something expecting the$Keys of some object.

New Features:

• We released a new implicit-inexact-object lint to detect when an inexact object is used without explicitly adding... to the
  end of the props list. Seehere for context.
• Function type parameters may now use default arguments. This is not yet supported by babel.

Notable bug fixes:

• Fixed a bug with ranges returned by autocomplete
• Fixed a bug where errors with bad locations reported over the LSP could cause the editor to clear all errors.

Misc:

• React.memo andReact.lazy now both allow you to specify an instance type viaReact.AbstractComponent.
• Various performance improvements to union types.
• Various libdef fixes and improvements.
• Various improvements to error positioning.
• The recursion limit is now configurable in the .flowconfig viarecursion_limit. Most projects will not need to override this value.

Parser:

• Forbid private fields named#constructor
• Fix duplicate private class field validation for getters/setters
• Fix parsing of private getters and setters
• Function type parameters may now use default arguments.