After the Major Digimon War; where the evil empire captured civilian and rebel digimon and corrupted their data into loyal Troopmon to continually add to their numbers, only barely stopping the escalating takeover by blowing up their corruption factories; all Troopmon were either destroyed or contained to try and restore them back to their original selves. This had varying results.

Bit the Impmon doesn't remember his old self, but he doesn't really care; His data was reconstituted into a form that was stable and now lives in the Convergent Landfill, a huge dump connected to all nearby regions. He now spends all his time toying with his junk... or well the Digital World's junk. Hanging out in all the grease and gunk has given him a glossy finish.

There ya have it, just a reassembled Garbage boy who'll chill with you as long as you don't steal his scrap.