Comment by maerF0x0

    DEFAULT_BRANCH=$(git remote show origin | sed -n '/HEAD branch/s/.*: //p')

    git branch --merged "origin/$DEFAULT_BRANCH" \
      | grep -vE "^\s*(\*|$DEFAULT_BRANCH)" \
      | xargs -r -n 1 git branch -d

This is the version I'd want in my $EMPLOYER's codebase that has a mix of default branches