I can't say there's a deck that's considered outright unsportsmanlike to play in-format right now, but there are decks that seem to be more disliked than others to play against
Lock decks, like Trevenant/Accelgor are the first thing that come to mind. Many players don't like the idea of a deck which, with the right combination of cards, simply make it improbable for the opponent to win from an early point in the game.
Mill decks (which you described) are genuinely uncommon nowadays, as there are no strong milling cards. However, the Durant archetype from previous formats did frustrate many players due to the games essentially turning into a race against time. I personally don't find mill decks to be too annoying, but that may be due to the current lack of consistent mill in the format.
Before the rule changes, decks which could do a lot of damage on the first turn due to a strategy called "donking" which involved knocking out your opponents only pokemon (especially against decks playing low-HP basics, like many evolution decks do) before they could even make there turn and play down more pokemon. This is often unsatisfying to the loser, or even both players, since very little actual "play" occurred.
All in all, the strongest strategies in the game at the moment are the most direct. Abusing alternate win conditions, like deck-out, is so uncommon that it's not really a problem. You can go ahead an use it to see your success. No matter what, there will always be cheap wins and sore losers, so it's best to just keep on playing.
Just remember to have fun.