Is the queens gambit declined getting more popular than the slav? 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.
It seems to be that the QGD is regaining some popularity at the top level due to theory advancing. I don't think that says anything about the overall popularity of the two openings though (i.e., including all players).

The increase in engines' powers have made playing the semi-slav a very treacherous choice. There's a lot more that super GMs need to know now in order to stay in the game. Meanwhile, the slav can result in Black getting squeezed, and in general super GMs avoid openings like this. The QGD can be thought of as a safe "middle-ground", in which Black can gain equality with a bit of precise play. This is the kind of Black opening that many super GMs tend to like nowadays.

Overall though, none of these openings have lost any validity at the non-top level.