Wow I did not appreciate that modern interconnect signals are no longer strictly 0/1 digital. Apparently the new 400 Gb ethernet standard also uses PAM4. Is there some underlying shift in process technology that is pushing things in this direction?
The first line is a typo surely? Did they mean PCI? PCIe came out in 2003 which is just shy of 2 decades.

> The PCI Express® (PCIe®) architecture has served as the backbone for I/O connectivity spanning three decades

How does this stay compatible with PCIe generation 5 and earlier while the encoding changes from interleaved digital (128b/130b from 3.0 to 5.0) in to PAM?
I can remember when ISA bus (at an awesome 4.77Mhz) was king. We've come a long way since.