check out some GM tournaments from last years and you will get the feeling that every other opening is rare.
Capablanca
I think at least half of all the variations were sometimes played by him. But I don't remember any of them has its name from Capablanca.
Okay. I still don't get what the OP is getting at then. Is the OP asking whether a specific variation of the queens gambit is sound or not in regards to modern theory, or is he asking if lookin over Capablanca's queens gambit games is a good way to learn the modern theories on the queens gambit opening?
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Is his queens gambit outdated, or does it still work?