While the base online game requires a membership, there are ways to enjoy Tetris 99 offline:

You can purchase this one-time DLC to unlock offline modes like Marathon and CPU Battle . Once you own this, you can play those specific modes without an active NSO subscription.

This fast-paced battle royale reimagines the classic puzzle game, pitting you against 98 other players in a fight for survival. Here is everything you need to know about getting the game legally and for free on your console. How to Download Tetris 99 for Free

Tetris 99 Switch : How to Get Your Battle Royale Fix (Official & Free)