Deleting a branch should not be so easy. There is even no warning before. Furthermore it is impossible (or I couldn't find) to find out who deleted a branch in the history of master branch. If a collegue deletes your branch accidentally, it is difficult to find out who did it and recovering is also very difficult. Finally I have three suggestions :
1- Strong warnings before deleting a branch
2- Registration of the executer in the master branch
3- A clear instruction to recover a deleted branch (from a local copy)
I would vote for the third point: Clear instruction how to recover an accidentally deleted branchline.
Thanks for the comment, Tom.