Pierre, when a branch that exists locally is deleted from 'origin' and I fetch/pull (in this case "QuietLog"), that's how it looks for me:

I'm using SmartGit 17.0.2 #10105, but this feature has been around for a while.

SmartGit already marks branches that don't exist in origin (have been deleted) in red. Maybe have a user-defined threshold for how old a fully-merged branch should be before it is marked in a different color to indicate it can be deleted. Let's say, any branch that is fully-merged into develop and it's last commit is older than two weeks will be marked in this way, so you know it's probably safe to delete.

Alternatively, you could have a separate "Clean up old branches" UI that can show fully-merged branches and how long ago their last commit was ("27 days ago") and have a checkbox next to each other and a "Delete selected" button. It might have additional controls to show old un-merged branches too ("abandoned"). It could also show un-merged feature branches that have strayed too far from develop that need integration ("4 days old, 67 commits behind develop").

But I think the core need to cleanup fully-merged, forgotten branches will give the most value.