Iirc, most copper lines had a 50kb cap, making 56kb modems liars.

I can't prove this but I somehow remember peaks of up to 48 kilobits per second though never sustained.

I remember being amazed to see download resume right in the browser even as late as 2009 (I was only on dial up u til about 2006).

IIRC limit isn't the copper, it's the CO interconnects with high/low frequency cutoffs, the same copper was used for 1.5Mbps synchronous DSL. For very short runs, 50Mbps VDSL