For S3 specifically you want to use Glacier. It's made for long term storage and is very, very cheap to store in.
Be warned though that restoration takes special procedures, time, and can be expensive. So Glacier is most definitely a place for storing stuff you hope you'll never need, not just a cheap file repository.
The Glacier fees for retrieving data in minutes are incredibly awful, so take that into account. Count on waiting 12 hours to get your stuff for cheap.
Glacier Deep is the cheapest option. It does come with a catch that there's a minimum of 180 days commitment for their infrequent access tier. Last time I checked, the cost for US-East-1 is roughly like this:
At $0.00099/GB/month, it would cost ~$12/year to store 1TB.
Retrieval cost is $0.0025/GB and bandwidth down is $0.09/GB (exorbitant! But you get 100GB/mo free)
So, retrieving 1TB (924GB chargeable) once will run ~$85. I've also excluded their http request pricing which shouldn't matter much unless you've millions of objects.
For the same amount of data, Backblaze costs ~$60/year to store but only $10 to retrieve (at $0.01/GB).
I suppose an important factor to consider in archival storage is the expected number of retrievals, and whether you can handle the cost.
Be warned though that restoration takes special procedures, time, and can be expensive. So Glacier is most definitely a place for storing stuff you hope you'll never need, not just a cheap file repository.
The Glacier fees for retrieving data in minutes are incredibly awful, so take that into account. Count on waiting 12 hours to get your stuff for cheap.