In Branch Permission - "Allow deleting this branch" checkbox only appears once a user is added into Write Access, or else it wouldn't appear.

There's no way to edit branch permission to configure it to prevent branch deletion in the new UI; a user will need to be added into Write Access first, and then the checkbox will appear.

Workaround for this issue:

• Edit Branch Permission
• Add any user to "Write Access". (It should show the two options - Allow rewriting history and deleting branch)
• Uncheck the option
• Click Save
• It should resolve the issue.