Criticisms of the Queens gambit declined

why dont you just play the slav?

One reason out of many is that Black is much more flexible after 2...e6- e.g. after 3.Nc3 he has no less that five entirely viable moves already, which lead to different strategies: 3...Nf6, 3...Be7, 3...c6, 3...a6 and 3...h6. All five of them have been played at the top level, and white has not a clear-cut way to a symbolic edge in any of them.

2...c6 in comparison, while a mighty good move, is more limited. I have been playing the classical Slav setup (involving ...dxc4 and ...Bf5) for years, but gave up on it as the positions resulting from 2...e6 are more dynamic AND objectively easier to handle.

The first problem of the slave [in my opinion] is the exchange variant ( 1.d4 d5 2.c4 c6 3.cxd5 cxd5) because it can be difficult to win with Black even against weaker opponents.

After there is 1) d4 d5 2) c4 c6 3) e3 that can annoy because of a Qb3 as soon as the Bishop goes to f5. Example :

a : 1) d4 d5 2) c4 c6 3) e3 Bf5 4) Qb3

b : 1) d4 d5 2) c4 c6 3) e3 Nf6 4) Nf3 Bf5 5) Qb3

3. ..h6, I've never seen ( from White's POV ), what are some critical games in that line ?

Depends on your definition of "critical". So far Nihal Sarin and Fedoseev are playing it pretty frequently, and occasionally, some patzers like Carlsen, Caruana, Artemiev, Fedoseev, Deac...

It may at first sight look like a waste of time, but on practice, it leads to QGD structures where Black has very clear-cut plans which can be employed with little theoretical knowledge (something which may change when the line becomes more popular).

White too can just play chess, provided Bf4 lines are familiar, but in essence it looks to me like a "test", Black is saying if I won't let you play Bg5, will you be able to play on?

QGD imho can only be played as a broad system, meaning White needs some familiarity with positions from all of Exchange, Bf4 and Bg5.

Depth of understanding will vary of course, but one better not just single out a single line to play the QGD effectively. Same for Black, it makes sense to play 2-3 QGD lines.

Maybe this is controversial but believe it to be true for QGD, while eg for the White side of a Grunfeld or a QGA I've only bothered to study one line.