Seems like it. All of Nintendo's games (like Smash Bros, Splatoon, Mario Kart), as well as third party games that rely on Nintendo's infrastructure (like Crash Team Racing) require a subscription. The only online games that don't require a subscription are games that don't rely on Nintendo at all, like Fortnite.
If you look at the Nintendo Catalogue on the Australian Nintendo website, New Horizons has the Switch Online logo, since they're required to display it on all titles that offer subscription perks, so it's safe to say that visiting or hosting will require a subscription.
Yeah I don't play Splatoon but I've heard it's like the best first party game for online service. Smash is decent usually, for me anyway. I just wish Mario Maker's had some improvements made because it could be so much funner. I love it, I just wish they could tweak it a bit.
HELP WANTED: Making a text RPG for the Forum Games board. PT for details!