Actually mod support on the xbox has been around for a lil bit. It was first introduced with fallout 4. The achievement system is turned off when an unofficial mod is in play, however there are also some official mods that will allow tracking and progress with the achievement system. But it has been some time since I last toyed with it.
Still, it was really neat. We could listen to different songs on the pipboy, had some unique armor and weapons added in. If I remember correctly there was even some quests with one of a kind power armors as the reward at the end of said quests....Of course god mode and whatnot as well.