Video evidence: https://youtu.be/_nF4kXCBvtM
Access to games’ DLC is managed on a per game basis, so if you delete DLC, you have to go back through the game itself to redownload DLC. You can’t do it in the eShop itself. (I just tested this with Mighty Gunvolt’s DLC to verify this.)
Smash does this a little differently – it makes you connect to ITS online servers before you connect to the eShop.
And because Nintendo just shut down online services for the 3DS & Wii U tonight, you can’t connect to Smash online, effectively meaning that all DLC you PREVIOUSLY PURCHASED is no longer available. (Only for the 3DS version; I can’t speak for Wii U)
Thanks Nintendo for not caring about games preservation outside of NSO rereleases.