Mario 3D All-Stars can still be redeemed after it goes off sale on March 31st
Mario 3D All-Stars may go off sale on March 31st, but you can still download your copy later if you get hold of a redeemable retail code.
The collection was marketed as a special limited edition for the Super Mario 35th anniversary, and will be removed from the Switch eShop once the promotion is over.
Redemption: Retail codes will still be redeemable after the game is removed from the store, and of course those who already own Mario 3D All-Stars can keep playing the game to their heart's content. Only the store listing will be removed from the eShop on March 31st.
Too physical: Nintendo will also pull the 35th anniversary merchandise from its retail locations, and servers for the battle royale Super Mario Bros. 35 will also be turned off on April 1st, effectively killing the game since it won't be playable without them.