/vr/ - Retro Games

In the strictest terms of "downloadable" content, the earliest example that comes to mind is Hudson's GB KISS technology. It was integrated directly into a select few Game Boy games, like Pocket Bomberman and Nectaris. It used an infrared connection that allowed user-created content to be shared between cartridges, and additionally functioned as a text message system. The content was stored via a battery backup just like SRAM data.