You can find out which branches have been merged into a branch by running the below:

$ git branch --merged origin/master
* master
  feature/some-branch
  feature/some-other-branch

This will also print out master as part of it, so we can then pipe that into grep to match results without master:

$ git branch --merged origin/master | grep -v master
  feature/some-branch
  feature/some-other-branch

Obviously, you won't want the name master in your branch name, otherwise it'll be excluded too. You can fancier with regex here.

Now we can take this result and pipe it into xargs, to run git branch -d passing the output as arguments:

$ git branch --merged origin/master
  | grep -v master
  | xargs -r git branch -d

I'm splitting up the command onto separate lines for readability. The -r will only run the git command if there is output.

I've noticed this doesn't always delete all old branches, but it can help. For example, perhaps you had a branch that was a work in progress, or a spike, with changes that aren't merged - this won't be picked up.

You can do one more trick to detect old branches that have no upstream branch anymore:

$ git branch -vv

This will list branches without an upstream branch with gone. Those can generally be deleted too.