title: GitLab CI trigger pipeline when branch/tag is removed
body: <p>I have a Git repo. GitLab CI is configured to deploy artifacts to orphan branch 'artifacts/$BRANCH'.   </p>
  
  <p><sub>(It may be that is not a best way to store artifacts, but it is convenient enough for that project purposes)</sub></p>
  
  <p>I want to detect a moment when BRANCH is deleted (merged&amp;removed), and then run CI job to remove corresponding <code>artifacts/$BRANCH</code>. I looked for a solution in docs, found keywords <a href="https://docs.gitlab.com/ee/ci/yaml/#onlyexcept-basic" rel="nofollow noreferrer"><code>only</code>/<code>except</code></a> but there was nothing suitable.</p>
  
  <p>At the moment I have the only idea to run clean up job with every pipeline and/or scheduled. A really cool solution would be 'trigger pipeline when branch removed'. Is this possible?</p>