Back when I used to study chess, like 10 years ago or so, all the text books and all the super GMs recommended either the slav or the semi slav against queens gambit, they said it's better than the queens gambit declined.
Moreover all recommended to avoid the queens gambit all together because it doesn't give white any advantage, and so many GMs just stopped playing d4.
Fast forward to now, I stopped studying chess, and I see everyone (by everyone I mean super GMs) playing d4 and black replying by playing queens gambit declined.
It's pretty much automatic for Caruana to play the declined and everyone is playing queens gambit, see the last FIDE championship and more.
What has changed? I feel like there's a big change in the text books nowadays, I don't know for sure, I haven't studied theory in ages.