Property does not exist on type JQueryStatic

but I would prefer making a $._data a valid method call.

The types intentionally don't allow internal API calls as the jquery team doesn't want you to use these methods. If you want to write unsafe code like this you are free to use $ as any as you have figured out.

If you want such unsafe access to be done safely you can extend the JQuery interface with new functionality

interface JQuery {
  _data: any; // Replace with your types
}