I have MK8 running in an emulator on my Deck with almost all DLCs. Online used to work using local LAN multiplayer (the emulator would redirect the LAN lobby to a server on the web). For multiplayer to work, all players must have the exact same version of MK8 running in their emulator.
Since the emulator’s development was shut down by a certain company, online multiplayer functionality needs some config changes to carry on working, and the lobbies are nowhere near as full as they used to be…
Edit: IIRC, there is a button you must hold while on the MK8 menu to enter local LAN mode
Yepp MK8D, with update 3.0.1 and the booster course pass DLC.
IIRC as long as the host player has the DLC, all other players can play on the DLC maps in the online session