On that day, physical versions of the game will be removed from all official Nintendo outlets, and the digital version will also be pulled from the eShop. It's safe to assume that retailers still carrying stock of the physical game will continue to sell off copies until they run out, meaning some physical units will still be available beyond Mario's destruction day, but what about the digital version?
Well, as it happens, Nintendo has confirmed that any retail download codes (as in, download codes purchased via a retailer, as opposed to a direct download from the eShop itself), will remain valid after 31st March (thanks, VGC).
Thankfully, this means that if you opt to buy a digital code over the next few days, you'll still be able to use it and activate the download – even though the product won't technically be on the eShop anymore. You might even find download codes still available at select retailers, just like physical copies, so there's still a glimmer of hope for anyone who still might be unaware of the game's removal after that date.
What a mess, huh?