That's an unhelpful description as the Slav and 2...e6 share little in common. Usually QGD means exclusively 2...e6.
It's debatable. I'm lazy to search extensive references but look here, for example:
https://www.365chess.com/eco/D10-D15_Queen's_Gambit_Declined_Slav_defence
In chess,com Openinings Explorer's Master Games, there were:
68,329 2...c6, the Slav Defense
52,819 2...e6, the Queen's Gambit Declined
17,870 2...dxc4, the Queen's Gambit Accepted
~6599 2...other responses.
It depends on the terminology cuz Slav can be considered as a part of QGD.
Good point! But then the discussion should be about Declined vs Accepted because after 1 d4, chess com's opening explorer shows:
452,896 1...Nf6
185,260 1...d5
...so the Queen's Gambit in any form isn't close to being "almost exclusively played" and 1...NF6, usually leading to Indian Defenses, is played 2.5 times more often! And the "top" GM's often play 1...Nf6 - including Caruana and Carlsen in the two Black games to date (one for each player) where 1 d4 was played in the current world championship match.
The two 1.d4 games have been Bf4 queens gambit declined, in current WC match. It is by far the most common choice of defending 1.d4 in world championship matches. It makes sense for every level of player to know something about it, because typical pawn structures occur in most straightforward manner, which occur in more complicated opening like Nimzo and Semi-slav.
No it can't :/
Come back to the kindergarten in order to learn some basic things.
https://www.365chess.com/eco/D10-D15_Queen's_Gambit_Declined_Slav_defence