How to export security and index rules from Firestore?

It's possible!

Run from CLI firebase firestore:indexes inside your firebase project folder.

Providing you have indexes already setup and logged into Firebase via the CLI too, you'll get a formatted JSON output for you to copy.

Example:

{
  "indexes": [
    {
      "collectionId": "teslaData",
      "fields": [
        {
          "fieldPath": "Model",
          "mode": "ASCENDING"
        },
        {
          "fieldPath": "Price",
          "mode": "ASCENDING"
        }
      ]
    }
  ]
}

Exported indexes can be re imported using firebase deploy --only firestore:indexes. Check following doc extract.

https://firebase.google.com/docs/firestore/query-data/indexing

You can also deploy indexes with the Firebase CLI. To get started, run firebase init firestore in your project directory. During setup, the Firebase CLI generates a JSON file with the default indexes in the correct format. Edit the file to add more indexes and deploy it with the firebase deploy command. If you only want to deploy indexes, add the --only firestore:indexes flag. If you make edits to the indexes using the Firebase console, make sure you also update your local indexes file.

I'm using Firebase CLI 4.2.1 if that helps, good luck :)

Edit: It's still working as of 9.6.0.

In your Firebase project folder execute this in the terminal:

firebase firestore:indexes > firestore.indexes.json

And it will save a file called firestore.indexes.json with your indexes.

You can then upload that file onto other Firebase projects.