Just a few days ago, Microsoft announced the news many had been waiting for: Call of Duty: Black Ops II for Xbox One via Backward Compatibility. This news sparked joy as players flocked to play the game again, taking many back to the classic Call of Duty times, back when people would talk in-game, join lobbies with friends and keep their boots on the ground.
Unfortunately, some keen players noticed one thing - League Play is unavailable. In fact, Activision purposely disabled League Play awhile ago, not anticipating that Microsoft would create a Backward Compatibility platform to reinvigorate the games' life. A player reached out to Activision to ask if League Play would be enabled again now that the playerbase has once again increased as a result of Backward Compatibility.
Hey there! Seasons will not be returning to League Play on BO2. Sorry for the inconvenience. ^MA
— Activision Support (@ATVIAssist) April 13, 2017
At the moment, it appears that Practice matches still function, however, you'll be unable to be placed into a season, with it stating that it is waiting for a placement, which never happens. If you're after the achievement that happens as a result of participating in League Play, you're also out of luck there.