0. Prune remote branches
   git remote prune origin
   Read more about git remote prune here

Deletes all stale remote-tracking branches under . These stale branches have already been removed from the remote repository referenced by , but are still locally available in "remotes/".

1. List local git branches
   git branch -vv will list all local branches along with some additional information such as their related upstream/remote branch and latest commit message

git branch -vv
#...
feature/some-local-only-feature cba8191 Some commit message
feature/some-old-feature cba2191 [origin/feature/some-old-feature: gone] Some commit message about some old feature
feature/some-active-feature wba2191 [origin/feature/some-active-feature: ahead 40, behind 10] Some active feature branch
#...

2. Filter git branches down to only those with deleted upstream/remote counterparts
   Next, we pipe the output from git branch -vv into grep 'origin/.: gone]'. This filters our list down to only lines that match the regex origin/.: gone] leaving us with

git branch -vv | grep 'origin/.*: gone]'
#...
feature/some-old-feature cba2191 [origin/feature/some-old-feature: gone] Some commit message about some old feature
#...
Read more about using regular expressions with grep here.

Random, useful grep tip : Invert Match
I just came across grep's -v (short for --invert-match) grep -v 'some pattern' that filters lines down to those not matching any of the specified patterns.

3. Pluck out branch names from output
   Piping that into awk '{print $1}' cleans up our output so we end up with a branch name per line.

git branch -vv | grep 'origin/.*: gone]' | awk '{print $1}'
#...
feature/some-old-feature
#...
This is because $1 translates into the 1st item in each line (items being separated by space(s)) which is the branch name. Example:

$1: feature/some-old-feature
$2: cba2191
$3: [origin/feature/some-old-feature:
$4: gone]
$5: First
$6: words
$7: in_commit_message...
...
4a) Delete the branches!
Next, we pipe our filtered down, cleaned up git branches list into git branch -d (short for --delete) and say our final goodbyes.

git branch -vv | grep 'origin/.*: gone]' | awk '{print $1}' | xargs git branch -d
The xargs portion results in the equivalent of manually typing out each of the branch names as subsequent arguments such as

git branch -d branch01 branch02 branch03 # ...
4b) Review/edit list before deleting
If you want to review and/or edit the list of branches before deleting them. One way to accomplish this is to pipe the branch names into your clipboard via pbcopy, paste them into your favorite editor, and then pipe them into git branch -d yourself. Example:

Pipe list into your clipboard via git branch -vv | grep 'origin/.*: gone]' | awk '{print $1}' | pbcopy
Paste into editor. Review. Edit as needed.
Copy the edited list back into your clipboard.
Pipe clipboard into commmand via: pbpaste | xargs git branch -d