I keep a command `git-remove-merged`, which uses `git ls-remote` to see if the branch is set up to track a remote branch, and if it is then whether the remote branch still exists. On the assumption that branches which have had remote tracking but no longer do are either merged or defunct.

https://gist.github.com/andrewaylett/27c6a33bd2fc8c99eada605...

But actually nowadays I use JJ and don't worry about named branches :).