Man, I’m glad my e-Reader still works.
For those of you who think I’m speaking gibberish, the e-Reader was an accessory Nintendo developed for the Game Boy Advance, and it was actually one of the first forms of DLC. Players would buy cards, which could then be read by the unit to unlock specific features for games (or levels). Since most of the content was ancillary and Nintendo games felt like full experiences at launch, it wasn’t really an issue.
Thankfully, one person out there transplanted the extra levels from Super Mario Advance 4(read: a remake of Super Mario Bros. 3) for everyone to enjoy, e-Reader or not. This reddit post has a full list, and I recommend checking them out.
Super Mario Bros. 3 e-Cards – All levels redone in Mario Maker [reddit]