How can I refresh the list of remote branches in my Visual Studio 2017 Team Explorer panel?

If you want Visual Studio 2017 to ALWAYS prune on fetch (I do, I add and remove a LOT of branches for UI work), once you install 3rd party Git (can do from Settings within Team Explorer, Git section) there will be a new option in Global Settings (also in Git section) to 'Prune remote branches during fetch' which you can set to True. I did not have this option before updating my VS to 15.5 (I was on 15.0).

You can Fetch in Visual Studio Team Explorer.

View => Team Explorer

In your local repo directory, you should use git fetch -p (or git fetch --prune) command. Then you will find the deleted branches from remote won't showed in remotes/origin in VS Branches panel.

This is because git fetch won't check the tracking references exist or not from remote repo. But for git fetch -p, it will check if the tracking references exist or not and delete non-existing ones before fetching.