New PCB materials (Megtron 6 or equivalent) are now required for M.2 slots. The spec forces OEMs to move away from "daisy-chain" M.2 topologies and implement dedicated for every Rev 5.0 M.2 slot.
This is the most heavily revised section. For the first time, the M.2 spec directly references for transmitter and receiver equalization. Specifically: pci express m.2 specification revision 5.0 version 1.0 pdf