I personally don't really need this feature, but a few artists in our office have recently switched over to Fork from SourceTree, and have told me they get anxious about accidentally deleting a remote branch along with their local branch since the checkbox is quite prominent in the delete dialog.

The ideal solution for this is probably a setting on the remote server to forbid deleting of remote branches via a git client, to eg. force you to delete it through the web client or via pull request being merged (after which you could then delete your local branch).
But as a client-side safeguard, it could be nice to have a per-repo option in Fork to always disable that checkbox so that they never have to worry about clicking it accidentally :)