You will have to trace the pipes going into it, to make sure, but, it looks like the pipe for your main plumbing and sewer line.

It's your main stack that goes to the sewer.

Thanks, there are no lines coming in or going out of the pipe, just capped junctions/clean-outs. I know the septic system was replaced around 5 or 6 years ago, and all the sewage pipes are now PVC.

Do you know if it would be typical that the old stack would still be connected to the septic, or would it be safe to cut and cap it in the attic?

I think you need to call a reputable plumber and come and look, just in case it is still in use. Its impossible for us to know if it is or isnt connected to anything just by pictures. Its pretty important that you know exactly before you go destroying it.