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.
I'm not going to unsubscribe for the Switch life. Plus, even if I did, I won't be playing these courses beyond 2023 probably. When I've played enough of a game I usually put it down and don't pick it back up. I got my fill of AC happy home paradise already. I'd have to have played AC paradise for years to get my money's worth out of buying the dlc, I play games generally for a set period of time and then move on never looking back (except MMO's). Subscribing is a far cheaper option to play all these games (NES/SNES/N64/genesis/dlc for modern games) for a player like me. I'm one and done 99% of the time. I would have easily spent more than $60 in 2021 on all the retro games and dlc that came to NSO, never to play those games again, so might as well just subscribe and have more money in my bank account.