Just only on Online matches, as long as at least one person owns the DLC. (So you need at least the base NSO sub)
Mario Kart 8 anywhere, anytime!
With Nintendo Switch, fans can enjoy the definitive version of Mario Kart 8 anywhere, anytime, even with up to eight friends in local wireless multiplayer.
www.nintendo.co.uk
This is... actually pretty generous of Nintendo. I was expecting separate DLC/Non-DLC matchmaking like they did on Wii U, but this is considerably better.Courses from wave 1 of the Mario Kart 8 Deluxe – Booster Course Pass can be played from March 18th locally or online in Friends and Rivals races, even if only one player owns the Booster Course Pass or has access to it as part of a Nintendo Switch Online + Expansion Pack membership. From March 22nd, courses from wave 1 will also appear in the Random selection when playing Global or Regional races with anyone online.