The initialization sequence for PCIe 6.0 is unique. FLIT mode requires new training sequences (TS1/TS2 Ordered Sets). Developers need the PDF to code the "Link Training and Status State Machine" (LTSSM) correctly to negotiate down to 5.0 or 4.0 if the link is unstable.
Despite the radical shift in signaling, the specification maintains strict backward compatibility. Legacy PCIe 1.0 through 5.0 devices will work in PCIe 6.0 slots, protecting existing hardware investments. pci express base specification revision 60 pdf
