I think the biggest change from last year is that teams are really starting to dissect and prepare for IT more than ever in his career. He's a bad defender but now, teams are game planning more than ever to exploit him. He's a terrific offensive player which is why teams are game planning more than ever to stop him. Teams are treating him like a superstar.
Last year, teams took an average of 8.2 FGA per game while guarded by Isaiah.
This year they are taking an average of 7.0 FGA per game while guarded by Isaiah. So that doesn't suggest that they are specifically targeting Isaiah in particular, any more than last year.
One big difference against the Celtics (not just Isaiah) is that teams are taking a TON more 3PT shots. And making them. They have made 11.2 threes per game against the Celtics this year, a difference of +3.4 made threes per game over last year. Those additional threes aren't all coming against Isaiah.
They are also getting to the FT line at a ridiculous 30.3 FTA/game rate, up +4.3 per game over last year. The Celtics are getting whistled for 24.7 PF per game, up from 21.9 last year. Given that Isaiah's PF/gm rate has only gone up +0.7, you'll have to include other players in the blame list for that as well.
When you look closely, I would say that teams ARE targeting specifics with the Celtics. In particular, they are attacking our reluctance to switch on perimeter picks and purposely stepping out for the open three. And when we adjust by showing more with our big, because we are missing Horford's high P&R D, they are attacking the paint, drawing the fouls.