Zenobius

git fetch --prune;
pruned_branches=$(git for-each-ref --format '%(refname) %(upstream:track)' refs/heads refs/remotes/origin | grep gone | cut -d " " -f 1 | sed 's/refs\/heads\///');
for branch in $pruned_branches; do
  git branch -d $branch;
done

or for you one liner maschocists:

git fetch --prune; git for-each-ref --format '%(refname) %(upstream:track)' refs/heads refs/remotes/origin | grep gone | cut -d " " -f 1 | sed 's/refs\/heads\///' | xargs git branch -d;

Explanation

git fetch --prune - Fetches the latest changes from the remote and prunes (deletes) any remote-tracking references that no longer exist on the remote.
git for-each-ref --format '%(refname) %(upstream:track)' refs/heads refs/remotes/origin - Lists all local and remote branches and their tracking status.
grep gone - Filters out the branches that no longer exist on the remote.
cut -d " " -f 1 - Extracts the branch name from the output.
sed 's/refs\/heads\///' - Removes the refs/heads/ prefix from the branch name.
xargs git branch -d - Deletes each pruned branch.

🙇