I don't think an automatic duel-warner should be legal in any way (like maybe if you had to manually ask it who is in your duel range - fine with me all it's doing then is saving clicks or displaying information you already have in a better way), but it's technically in the same category as a chat colour text script, so it makes the whole thing a joke and up to the players own interpretation and/or morality towards cheating in a browser game which has now fundamentally changed it's design to be even more grind focussed.
"Prohibition" has also never worked in the history of humanity, so just tell us which features are good to script, or make your own script features but better if you don't want people to require them for gameplay - i think it's called development
But, no, the game is "feature complete" so scripting is the only option to improve the terrible ui.
However, it doesn't really work like that. I tested it. It often lags and send notifications up to 5-6 minutes later than it should and it messes me up.
I suspect the reason is it was designed to only check every ~6 mins to not lag the server as much, which is good practise