In a response to Reddit Steam community members, Valve's own Gabe Newell confirmed that Steam Deck console indeed uses an M.2 NVMe slot, but it is a 2230 form-factor one, which is pretty hard to find. In two separate emails, Newell confirmed that it is both M.2 2230 form-factor slot, even in the base $399 version which comes with 64GB eMMC storage, but official specifications from Valve say that these are "not intended for end-user replacement".
The M.2 2230 is a hard thing to find on the market, and most come as salvaged units from other devices. KIOXIA, a well-known manufacturer, has a few of these in retail, and in Europe, these can be found for around, €40 for the 128GB, €60 for the 256GB model, €100 for the 512GB one, and around €190 for the 1TB model.
It is possible that upgrading the Steam Deck to your own M.2 2230 SSD might not be too hard but so far, which will be a great thing for those that want a cheaper way to upgrade the console. On the other hand, we still haven't seen the insides of the Steam Deck, so it might not be as easy as some think.
The Steam Deck is launching in December 2021, and scalpers are already doing their thing, with some Steam Deck offers on eBay reaching as high as $800 for the 256GB version. The cheapest version of the Steam Deck starts at $399. The other two storage options go for $529 and $649.