You may not be too comfortable paying for the Nintendo Switch Online service, even though it is only $20—because it is not all that good of a service, to be perfectly honest (and I’m still being kind here). While Nintendo has committed to improving it, right now, maybe you don’t want to encourage them by spending money on it.
Thankfully, if you are a Twitch Prime subscriber, you get to have your cake and eat it too. Twitch Prime subscribers are getting one year’s worth of Nintendo Switch Online for absolutely free; you get three months right upfront, and then you can claim the remaining 9 months 60 days later (presumably Twitch wants to make sure you won’t just take the free subscription and cancel your account with them).
With Nintendo Switch Online, you can play the NES games Nintendo puts out for the service every month, as well as get access to cloud saves on Switch, games such as Tetris 99 (which is exclusive to the service), and obviously, play Switch games online in general. And you don’t even have to pay the $20 for it.