Historically, PCIe used 128b/130b encoding (PCIe 3.0–5.0), which means for every 130 bits sent, 128 were data and 2 were overhead for frame synchronization.
For serious hardware professionals, downloading and studying the official is non-negotiable. It holds the keys to designing next-generation AI accelerators, terabyte-capable SSDs, and high-performance computing clusters. pci express base specification revision 60 pdf
The PCIe specification has always prided itself on backward compatibility. A PCIe 6.0 link will fall back to the highest common supported speed. Historically, PCIe used 128b/130b encoding (PCIe 3