M.2 2230 SSDs are hard to find
According to Gabe Newell's response to Reddit Steam members, it appears that the Valve Steam Deck features an M.2 NVMe 2230 slot, making an upgrade possible, but Valve says it is not intended for end-user replacement, even if you can find it.
Shipping in December and starting at $399
Valve has now officially announced its handheld console, called the Steam Deck. Based on a custom AMD APU with Zen 2 CPU and RDNA 2 GPU, the Steam Deck should pack enough punch to run games on a 7-inch screen when it launches in December 2021, with a starting price of $399.