Although we will usually receive advance notification of new games releasing as part of Nintendo Switch Online subscription service, sometimes Nintendo will add a new game and tell us about it after the fact, and their latest surprise offering is Rare’s Killer Instinct Gold. You’ll need to be a member of the Expansion Pack service to be able to access this one from the N64’s retro library.
If you want see some footage of the game before giving it a try yourself, you can check out the announcement trailer from Nintendo of America below:

Finally!
😯
OMG !!!!
I’m going to have to subscribe now. This brings back good memories.
It’s a good game in its own right, but it was the least good Rare game that I was looking forward to from Rare and the N64 era.
I’m mostly still wanting DK64 and Diddy Kong Racing, not entirely looking forward to Conkers Bad Fur Day, Star Fox Adventures, Mickey’s Speed Way USA.
Still would be nice to see DKC 1 2 and 3, for GBA.
I’m sure we’ll never get it, but Mickey Speedway USA was so freaking great. I swear I played that more than I played Kart64.
Major kudos to Microsoft. Finally… Killer Instinct GOLD is finally on NSO Expansion Pass