https://devblogs.microsoft.com/typescript/announcing-typescript-4-9-rc/

This issue is just to track all of the new features and their implementation state in this project.
As with all releases, we will not necessarily to support all features until closer to the full release when everything the features are stabilised.

Please be patient.

✅ The satisfies Operator (#5717)

const palette = {
    red: [255, 0, 0],
    green: "#00ff00",
    bleu: [0, 0, 255]
//  ~~~~ The typo is now caught!
} satisfies Record<Colors, string | RGB>;

This will require AST changes

✅ Auto-Accessors in Classes (#5926)

class Person {
    accessor name: string;

    constructor(name: string) {
        this.name = name;
    }
}

This will require AST changes.
This feature is based on the stage-3 decorators proposal.
We will need to implement the defined ESTree AST

✅ substitute Replaced With constraint on SubstitutionTypes

We will need to double-check our codebase for usages of the old property.

✅ lib.d.ts Updates

We will need to regenerate our types within scope-manager.

Other changes with no impact to us

• Unlisted Property Narrowing with the in Operator
• Checks For Equality on NaN
• File-Watching Now Uses File System Events
• "Remove Unused Imports" and "Sort Imports" Commands for Editors
• Go-to-Definition on return Keywords
• Performance Improvements
• Breaking Changes
  • Better Types for Promise.resolve
  • JavaScript Emit No Longer Elides Imports
  • exports is Prioritized Over typesVersions