How to send a slack notification at end of workflow

For scenarios where you are using the Slack orb and want to send a notification if a workflow itself has been successful or failed -- you would need to implement a notification per job.

For workflows that run sequentially, you can anchor the Slack notification and apply the anchor to each of the jobs in the workflow via post-steps -- except the last job that is run. For that job, you would set it to the standard slack/notify to catch both successes and failures. Below is an example of how this may look with Slack orb version below 4.0.0:

version: 2.1

slack-fail-post-step: &slack-fail-post-step
  post-steps:
    - slack/status:
        fail_only: true

orbs:
  slack: circleci/slack@3.4.2

jobs:
  build:
    docker:
      - image: cimg/base:stable
      
    steps:
      - run:
          name: Success
          command: |
            exit 0
  last-job:
    docker:
      - image: cimg/base:stable
      
    steps:
      - run:
          name: Success
          command: |
            exit 0
  
workflows:
  version: 2
  workflow:
    jobs:
      - build: *slack-fail-post-step
      - last-job:
          requires:
            - build
          post-steps:
            - slack/status

However if your workflow jobs don't run sequentially, you will need to create a new job that contains the slack notification and require that the job has run all the previous jobs.

Here is a sample of how you could set this up if you use the Slack orb version 4.0.0 or above:

version: 2.1

slack-fail-post-step: &slack-fail-post-step
  post-steps:
    - slack/notify:
        custom: |
          {
            "text": "",
            "blocks": [
              {
                "type": "section",
                "text": {
                  "type": "mrkdwn",
                  "text": "❌ *Failure* #${CIRCLE_BUILD_NUM} `${CIRCLE_PROJECT_REPONAME}` on `${CIRCLE_BRANCH}`"
                }
              },
              {
                "type": "actions",
                "elements": [
                  {
                    "type": "button",
                    "text": {
                      "type": "plain_text",
                      "text": "View Job"
                    },
                    "url": "${CIRCLE_BUILD_URL}"
                  }
                ]
              }
            ]
          }
        event: fail

orbs:
  slack: circleci/slack@4.1.1

jobs:
  build:
    docker:
      - image: cimg/base:stable
      
    steps:
      - run:
          name: Success
          command: |
            exit 0
  last-job:
    docker:
      - image: cimg/base:stable
      
    steps:
      - run:
          name: Success
          command: |
            exit 0
workflows:
  version: 2
  workflow:
    jobs:
      - build:
          context: 
            - slack
          <<: *slack-fail-post-step
      - last-job:
          context: [slack]
          requires:
            - build
          post-steps:
            - slack/notify:
                custom: |
                  {
                    "text": "",
                    "blocks": [
                      {
                        "type": "section",
                        "text": {
                          "type": "mrkdwn",
                          "text": "✅ *Success* #${CIRCLE_BUILD_NUM} `${CIRCLE_PROJECT_REPONAME}` on `${CIRCLE_BRANCH}`"
                        }
                      },
                      {
                        "type": "actions",
                        "elements": [
                          {
                            "type": "button",
                            "text": {
                              "type": "plain_text",
                              "text": "View Job"
                            },
                            "url": "${CIRCLE_BUILD_URL}"
                          }
                        ]
                      }
                    ]
                  }
                event: always