How can I get notification only if a specific branch fails in CircleCI?

I wish to receive notification if the master branch fails in CircleCI.

There's a user request to add this feature, but it was closed without comment - so I don't know if Circle added the feature or didn't add it.

I can see there's filter in the Configuration Reference, but these seems only exist on the entire workflow - I'd like to filter just the notify part.

config.yml is included below.

version: 2.1

orbs:
  node: circleci/node@4.0.0
  slack: circleci/slack@4.0

executors:
  docker-node-12:
    docker:
      - image: cimg/node:12.19

  full-app:
    resource_class: 2xlarge
    docker:
      - image: cimg/node:12.19
      - image: elasticsearch:6.4.3
      - image: circleci/postgres:10-ram
      - image: circleci/redis:6

jobs:
  install npm packages:
    executor: docker-node-12
    steps:
      - checkout
      - node/install-packages
      - persist_to_workspace:
          root: /home/circleci/project
          paths:
            - .

  run tests:
    description: Run App's tests
    # https://circleci.com/docs/reference-2-1/#docker
    parameters:
      filter:
        description: "Test filter for jest"
        default: ""
        type: string
    executor: full-app

    steps:
      - attach_workspace:
          at: /home/circleci/project

      - run:
          name: Run tests
          command: |
            cd /home/circleci/project; set -o allexport; cp .env.circleci .env; source .env
            npx jest --passWithNoTests --runInBand --logHeapUsage << parameters.filter >>

  notify:
    docker:
      - image: "cimg/base:stable"
    steps:
      - slack/notify:
          custom: |
            {
              "blocks": [
                {
                  "type": "section",
                  "fields": [
                    {
                      "type": "plain_text",
                      "text": "Oh no, master failed!",
                      "emoji": true
                    }
                  ]
                }
              ]
            }
          event: always


workflows:

  install_and_test:
    jobs:
      - install npm packages
      - run tests:
          requires:
            - install npm packages
          matrix:
            parameters:
              filter:
                - --testPathPattern='a'
                - --testPathPattern='b'
                

      - notify:
          context: slack-secrets