You can just go to Cory's mom's Twitter. She confirmed that days ago right after it happened. She was even asked not to talk about it before the show happened which speaks volumes of Britney's authenticity.
I'm sorry but there are greater chances of Brit singing a ballad live again and her next album smashing than her doing a promo tv performance ever again.
If I'm not mistaking, I think her very last tv perf was with toxic with Ellen and she slayyyyyyed. It's better to end with a high note IMO