Do you remember GB KISS? Probably not, since it was only released in Japan circa 1997-1998. Which is unfortunate, because this technology let you transfer saves and other data (inclding DLC!) between your PC and supported games.
Directly from the game cartridge itself.
*After plugging the IR receiver into your PC’s parallel port.
Well, GB KISS LINK was an infrared modem that allowed folks to connect their Gameboy (with a GB KISS cartridge installed) to a personal computer (running PC Windows 95). This allowed gamers to transfer data (i.e. game save states, custom maps created with the map editor, exclusive content downloaded from Hudson’s website, etc.) between their PC and Gameboy cartridge.Esteban (emphasis added)
GB KISS games could also transfer data between two Game Boys, similar to other titles that went on to use the Game Boy Color’s built-in IR reader (such as Pokémon Crystal.)
The linked article is chock-full of photos of GB KISS marketing materials and hardware, and spans multiple pages. It’s a fascinating read and quite in-depth!