Private identifiers are only available when targeting ECMAScript 2015 and higher (Angular 9)

I'm upgrading from Angular 8.2 to 9.1 and I'm trying to change private fields to be #fieldName: any as allowed by TS 3.8 instead of private fieldName: any. And I get the compiler error saying

Private identifiers are only available when targeting ECMAScript 2015 and higher.

tsconfig.json:

{
    "compilerOptions": {
        "module": "esnext"
        "target": "es2015",
    }
}

package.json:

"devDependencies": {
   "typescript": "3.8.3"
}