Valve may have cautioned Steam Deck buyers about repairing the handheld themselves, but you might not have to be quite so wary in practice. iFixit (which provides official parts for Valve hardware) has torn down the production model Steam Deck and discovered that it's relatively easy to repair, with a few notable exceptions. It doesn't require much effort to get inside, with clear labels for "basically everything." The design is modular enough that you can repair many elements without replacing or dismantling more components than necessary.
You can replace the SSD with a similarly small equivalent using one screw, and removing the display doesn't require much more than a suction cup and some heat. Valve is also keenly aware that Steam Deck owners may be worried about thumbstick drift — you can replace both sticks just by removing three screws. While drift won't necessarily be a real problem, you won't have to send in your machine if that problem ever crops up.
This isn't quite a self-repair paradise. Battery replacements are "rough," according to iFixit, with elaborate procedures that include draining most of the pack for safety's sake. That's a problem when the battery can drain in as little as 90 minutes. You may also want to be gentle with the USB-C port and microSD slot, as both are soldered to the Steam Deck's custom motherboard.
Despite this, iFixit found the Steam Deck easier to repair than some modern laptops. That might be heartening if you either prefer to fix devices yourself or hope to upgrade the storage. You might only need to send the Deck away for service if there's a truly serious failure.