The latest update for Nintendo Switch was just released, bringing the console up to Version 16.0.0. While the update includes the usual vague “stablity” enhancements, it also mentions a change to the detection of inappropriate Switch usernames. See below for the official wording:
User nicknames that cannot be used will be replaced with “???” which can be updated from the profile settings.
We now have a slightly better understanding of this change, according to Dataminer @OatmealDome. It seems that this update did slightly alter the way the Switch parses and detects usernames. The list of flagged words has been rearranged slightly, and some new ones have been added, including “sexually charged language, drug-related terms, words related to grooming, swears, etc.”.
If your username is determined to have any words Nintendo doesn’t like, it will be replaced with “???”, but you’ll be able to go into your settings and change it whenever you like.