git clean-stale-local [-n|--dry-run]
This contrib script is to be run inside a local Git repo. It looks for local branches merged into the default branch that track now-pruned remote branches on origin, and deletes these refs.
Yes, this assumes your main remote is origin.
No, it doesn't assume your default branch is master: it will look at your locally cached ref for origin/HEAD and, if there is none, will fetch that from the origin remote itself (failing that, it'll abort).
You can use the traditional -n / --dry-run flag to list the would-be removals without running them.