Narratively it's mildly disappointing, because there's definitely a lot of emotional baggage - like after Mystery Spot, Sam doesn't tell Dean about the months of Dean being dead and Sam being on his own. But we know, and so it's kind of amazing? ALSO HEY maybe Dean read about it in the Carver Edlund books. I think they probably resolve a lot of this stuff, where talking about it doesn't change anything but still feeeeelings, with fucking each other extra hard into the mattress with no explanation. I like it that way :))

once Dean knows that Sam's a vessel, I can understand why he wouldn't tell Sam about the croatpocalypse.
Hmmm yeahhhh. Though I feel like at some point he shared about it, because doesn't Lucifer say something like "it always ends in Detroit" when they meet up in Swan Song? Maybe Dean uses his opportunity to warn Sam away, you know? That seems to be the headspace he's in after the Croatpocalypse anyway - Zach shows him the future so Dean'll say yes, but instead Dean takes away the message of "look after Sammy".