I don't think that is true. I've entered the DLC shop (for smash) many times and it didn't delete it ever. Not even when I went onto the eShop. I went there to check if it installed right.I've done a lot of research into when DLC is deleted and when it isn't, and the closest thing I reached to a conclusion was that as long as you never enter the game's DLC shop, it won't delete your DLC. However, once you visit the game's DLC shop once, it'll delete your DLC whenver you go into the game's DLC shop or the eShop both.
Sent from my Browser on a New 3DS XL with A9LH