Nintendo has introduced integrity checks in Splatoon 2 since version 3.1.0. I've known about this for several months, but have kept silent about it since Nintendo did not seem to be actively banning people. Since an independent researcher (Khangaroo from Splatoon Modding Hub) has discovered this, I am now publishing this post. For Nintendo's sake, I won't be going into detail about how it works as of yet. It seems that bans are applied one day after the game flags you.
Beware: the game will flag you regardless if you use the mods online. Simply starting the game with edits is enough to flag you.
The following activites are probably still safe:
- Model edits
- Music replacement
- Text mods
- Save edits, provided what you are doing can be considered as legitimate
- and more...
Note that the above activites aren't guaranteed to be safe in the future.