The impression I get is that 2024 D&D is more concerned about "telling a story" rather than open, unscripted adventuring.
That said, WotC has done several micro sandboxes - Curse of Strahd, Tomb of Annihilation, and Rime of the Frostmaiden off the top of my head. Maybe even Undermountain as a megadungeon could be considered a quasi-sandbox.
So, sandboxing isn't quite dead, they just didn't bother to discuss it in the 2024 DMG. Their more recent (Shadows of the Dragon Queen, Phandelver, Eve of Ruin) seem to utilize a more path-like (though multiple paths) structure than a true sandbox, and I think for the sort of adventure books they are making those are easier as they have a beginning - middle - end structure.
As always, if you want to see more sandbox adventures from WotC, tell them. I'm sure they'd be interested in giving us what we want - it's money in their pockets.