Can you go ahead and set the FK_User_To_CONTENT_AUTHOR to ON DELETE CASCADE and then just left join the CONTENT_AUTHOR and User tables to the Content table? Since you have the Content record, you will still know that a User worked on it:
select
ifnull(u.UserID,0) UserID,
ifnull(u.UserName,'Expired User') UserName,
c.ContentCreated
from Content c
left join CONTENT_AUTHOR ca
on ca.ContentID = c.ContentID
left join User u
on u.UserID = ca.UserID
If the number of User records related to the Content is significant, I suggest creating an Expired User record of some sort and inserting it into the CONTENT_AUTHOR table as part of the User deletion process:
insert into CONTENT_AUTHOR(UserID,ContentID)
select u.UserID,c.ContentID
from User u
inner join CONTENT_AUTHOR c
on c.UserID = 2
where u.UserName = 'Expired User';
delete from User where UserID = 2;
It would be great if we could SET DEFAULT on that FK_User_To_CONTENT_AUTHOR key, but I don't think we can.