As Robert says, openings are the obvious change: new variations for the 3-3 invasion, re-evaluation of "old" joseki, rehabilitation of
shapes we were taught are bad, reduced popularity of
high concept openings, ... What can we say about the middlegame? I think there has been a real influence there too, but it's harder to describe in a few words.
To my mind, a big factor is that the new AIs are very good at
responding to an attack. By this, I don't mean "defending". For an amateur like me, if my stones are threatened with attack, my first instinct is to try and keep them alive. But there's a lot of other options. Sacrifice and build influence. Make a light shape and allow the possibility that part of the "group" is cut off. Tenuki and do something more important elsewhere. Of course we already know this. Most of us learn "light and heavy" some time in the single-digit kyu ranks. But knowing the concept and finding the moves to put it into practice are two different things. And I think AI is taking it to the next level. This is why moyo strategies are becoming less popular, and it's so much more common for games to break out into early and chaotic fighting rather than building a framework first.
One factor is that the AI sees the board anew on each move. It doesn't have an emotional attachment to previously played stones. So it stands to reason that it should be more prepared to make trades in a way that feels risky to humans. (Why shouldn't I take risks in a close position? If it's risky for me, then it's equally risky for the other person. But somehow it seems to go against instincts.)
To some extent this is an acceleration of trends that were already happening in the 1990s. So it's hard to pick out how much AI has given things a push and how much it would be happening anyway.
There's two other things I'm noticing when I review games with KataGo. It seems to play forcing moves earlier than was recommended a few years ago. I've always been taught to avoid pushing the opponent around unless I can see a clear benefit. Leave things open, because you don't know what options you might need later. But it's a fine line between leaving it open versus waiting too late and losing the chance entirely. Except that KataGo seems to be suggesting it's not such a fine line: just do it as soon as the chance comes up (usually, not always).
And I think the definition of "probe" has become a lot broader. I see KataGo saying "Why don't you just drop a stone into that area that looks like your opponent's settled territory? It costs nothing (they have to reply), and might provide some useful aji in the endgame." Again not a new idea, but happening earlier in the game and more often than I'm used to.
Sorry I'm too lazy to pull out specific examples from pro games today. I might come back here next time I notice something. Meanwhile, people can tell me where my opinions are wrong, and we'll have another interesting conversation :-)
I'll also go out on a limb and say that AI has
not changed the late endgame (the part where you're down to 6-point moves or smaller). Humans already have a good conceptual framework for that part of the game, and in slow time controls I think the top players have been able to get pretty close to perfect endgame play for some decades. Computers might be stronger in terms of being more consistent, less likely to make mistakes under pressure, but I don't think they've introduced any new concepts for this part of the game.