If you squash your PR before merging, then this alternative worked really well for me:

  git fetch --prune && git branch -vv | awk '/: gone]/{print $1}' | xargs git branch -D

Almost identical to mine, but you've got smarter awk use: `git prune origin && git branch -vv | grep 'origin/.*: gone]' | awk '{print $1}' | xargs git branch -D`

I think I probably copied this from Stack Overflow close to a decade ago. Seems like a lot of people have very similar variations.