https://wccftech.com/steam-deck-ssd-downgraded-newer-256-gb-512-gb-models/

Earlier this week Valve announced they plan to double Steam Deck shipments over the coming months, but it seems this increased production may come at a cost. While it’s been overlooked by most people, German website HardwareLuxx noticed that the Steam Deck website was recently updated to note that some 256 GB and 512 GB models will now ship with a PCI express 3.0 x2 SSD instead of a PCI express 3.0 x4 SSD. As you might expect, an x2 SSD offers around half the writing and reading speed of an x4 SSD. Here’s Valve’s exact statement on the matter…

256 GB NVMe SSD (PCIe Gen 3 x4 or PCIe Gen 3 x2*)