So this is not quite a game, but existing Amazon Prime members who link their prime account with twitch Prime can get Nintendo Online for free for 12 months (3 months to start but renewable after 60 days).
there are also 4 games for free:
kabounce
star vikings forever
the king's bird
snake pass
Not quite free given that you need amazon prime, but lots of people have prime already. Sorry if this post is not qualified as a free giveaway because it only sort of is.
It's fine I guess since Nintendo Online is kinda important (since nowdays online is paid on consoles and all). By the way, The King's Bird you mentioned is an interesting game for anyone curious, but can get boring. Don't know about the others...