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"
}

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