Firestore - Listen to specific field change?

You can't. All operations in Firestore are on an entire document.

This is also true for Cloud Functions Firestore triggers (you can only receive an entire document that's changed in some way).

If you need to narrow the scope of some data to retrieve from a document, place that in a document within a subcollection, and query for that document individually.

As Doug mentioned above, the entire document will be received in your function. However, I have created a filter function, which I named field, just to ignore document changes when those happened in fields that I am not interested in.

You can copy and use the function field linked above in your code. Example:

export const yourCloudFunction = functions.firestore
.document('/your-path')
  .onUpdate(
    field('foo', 'REMOVED', (change, context) => {

      console.log('Will get here only if foo was removed');

    }),
  );

Important: The field function is not avoiding your function to be executed if changes happened in other fields, it will just ignore when the change is not what you want. If your document is too big, you should probably consider Doug's suggestion.