I guess you will lose all 48 additional tracks as soon as you unsubscribe from NSO+, just like the Animal Crossing Happy Home Paradise DLC.
To be honest, this was the last thing I have expected almost 5 years after the launch of MK8D. I thought this DLC would come in 2018 or 2019 at the latest - but now??
MK8D already had the most courses of any Mario Kart - and we get another 48 tracks to double the content? Wow, I bet some tracks won't come in ages if you play online. Don't get me wrong, MK8D is a good game with the best physics to date, but it also lacks in long term motivation to play the game (you don't unlock anything anymore), there are no events, online lag just sucks, you have too many shells with 2 x 3 shells and 12 characters playing and you can't drive with your favourite character and kart because of disadvantage. Mario Kart Tour does all these things better, but the controls are horrible.
MK8DX is the Nintendo game with the smoothest online experience. Didn't ever have any lag.