This will take a while to do.

1. Make a new clone of your repo.

    git clone <repo> <repo>-bak
    cd <repo>-bak
   

2. Make a backup of all branches locally in case you fuck up something.

    git branch -r | grep -v '\->' | while read remote; do git branch --track "${remote#origin/}" "$remote"; done
    git fetch --all
    git pull --all
    # via https://stackoverflow.com/questions/10312521/how-to-fetch-all-git-branches#10312587
   

3. List all remote branches, oldest first

    git for-each-ref --sort=committerdate refs/heads/ --format='%(committerdate:short) %(refname:short)'
    # via https://stackoverflow.com/a/16961359/1225741
   

4. Add this to your .(zsh|bash)rc

    git-is-merged() {
        merge_destination_branch=$1
        merge_source_branch=$2
        merge_base=$(git merge-base $merge_destination_branch $merge_source_branch)
        merge_source_current_commit=$(git rev-parse $merge_source_branch)
        if [[ $merge_base == $merge_source_current_commit ]]; then
            echo $merge_source_branch
        fi
    }
    
    git-list-merged() {
        git for-each-ref --sort=committerdate refs/heads/ --format='%(committerdate:short) %(refname:short)' > /tmp/zsh-git-refs-temp
        while read line; do
            git-is-merged HEAD $(echo $line | awk -F" " '{print $2}')
            #echo $line
        done < /tmp/zsh-git-refs-temp
        rm /tmp/zsh-git-refs-temp
    }
    
    git-rm-merged() {
        git-list-merged | egrep --invert-match '(master|sprint)' > /tmp/zsh-git-refs-merged-HEAD
        while read line; do
            echo git push origin --delete $line
        done < /tmp/zsh-git-refs-merged-HEAD
        rm /tmp/zsh-git-refs-merged-HEAD
    }
   

Run git-rm-merged (it's non-destructive, it will echo out what it would do). Adjust the egrep to your liking to exclude branches that you want to leave in.

Remember to source the changed .zshrc with . ~/.zshrc after doing changes.