Can I delete a user without losing any data?

Yes, it is safe to delete a user. The historic information about the user will be kept, including which backlog items and tasks are assigned to the person.

Just note that, if you later add the person again, the new user account will be completely separate from the previous user account. So, you cannot resume the same user account after deleting it, or connect the two user accounts. It's not yet possible to deactivate a user without deleting the user.