Unintentionally removing branch when merging merge request

• The "Remove branch" option in a merge request is by default on (https://gitlab.com/gitlab-org/gitlab-ce/issues/14413), so it is easy to have the branch removed with intending to do so.
• How to ensure that users are aware of this action when they click the Merge button. What are UX improvements we can ,make in the merge request widget to prevent unintended branch removing?

Description

The checkbox for Remove source branch should be unchecked by default when you create a new merge request. I.e. it should be unchecked in the new merge request web form.

Future steps

• https://gitlab.com/gitlab-org/gitlab-ce/issues/34422 Undo removing a branch
• https://gitlab.com/gitlab-org/gitlab-ce/issues/34424 Default removing source branch in merge request for non-long-running branch

Related

Protected branches cannot be deleted. There is a UI bug, to be fixed in 9.5: https://gitlab.com/gitlab-org/gitlab-ce/issues/33264.