Topic: Are you playing Yu-Gi-Oh master duel?

@TheStardustWarrior actually no, Yu-Gi-Oh master duel does not require switch online membership to play it online. After logging into master duel you can go to your player profile and you will see a nine digit code, that code is your Yu-Gi-Oh master duel ID number. If you wanted to play against a friend all you and that friend would need to do is input that nine digit code to register with each other.

I believe it is due to the game running through Konami servers and not Nintendos to establish a connection to other players of master duel on other platforms (PS4, PS5, the Xbox series of consoles, steam, iOS, android, and of course switch.)