And what do they do if someone stores 75TB of stuff? "Unlimited" nearly always means "we will fire you as a customer if you use way more than anyone else".
CrashPlan has unlimited backup and never deletes files. And keeps all the old versions of the files. I don't know how they stay in business, given that somebody probably uses it to backup petabytes of constantly changing data. They have pretty slow upload/download speeds though so perhaps something like Amazon Glacier is involved.
I was just about to say that CrashPlan is restricted to the internal drive of a computer & is therefore limited to the maximum size of hard drives (e.g. 1-2 Terabytes for laptops), but apparently CrashPlan lets you backup external drives now too:
Having used CrashPlan's personal offerings for many years, I recall that they have always allowed the backup of attached storage either from hard drives or network mounts.