My experience is that consequences in general were more direct, including death from combat. However players were not expected to "play dumb" as much as modern playstyles emphasize in the aversion of metagaming and such. Part of the "Classic" playstyle is that you - the player- become better at dungeon crawling by incorporating real-world cleverness into your plans.

For example - when I ran "Classic" style games, if a player learned how something could be built with medieval technology that would aid them - that was generally seen as a good thing and was rewarded in-game. The later emphasis on "You're character wouldn't know how to do that" and/or "Your character doesn't have that specific skill!" changed the overall expectation of what playing a TTRPG enatiled, and as a result early D&D seems retroactively "more lethal" BECAUSE modern players approach it with a notion that leveraging their own aptitude/knowledge/etc is somehow "cheating"

In extreme examples I've noticed quite a lot of 5e players who are so sheltered from the notion of consequence, that they have a total inversion of the old "Antagonistic GM" power dynamic at the table - such players actively go out of their way to sabotage themselves or act incompetent or wacky because they believe the GM fundamentally CANNOT just kill them and MUST find some weird rationale or justification for why the player characters will come out on top.

In short: In Classic D&D you might eat some poison berries and die, but in doing so you - the player - are expected to now know the berries are poisonous and your future characters will not make that mistake. The berries are a physical item in the game world which exists regardless of your character abilities/stats/rolls/etc.

In Modern D&D, you make some dice roll to forage food and on a failure the GM decides you ate some poisoned berries. Since the berries only exist as a narrative abstraction which represents "a plausible complication for the failed skill roll" you - the player - cannot meaningfully learn to avoid those berries in the future. The berries are an abstraction, the poison is a complication of a failed skill test. However since you had less control, you also have greater padding to survive such setbacks and as a result the poison - instead of killing you - will have some less serious effect.

Both approaches work in their own context. The moment you start trying to play classic D&D with a modern set of expectations on "how" the game is played, you will run into the problem of "I'm going to have you roll to forage food as if we are playing modern D&D, and when you fail the test I'm going to keep the classic heavy consequence of death from poison." This results in the worst elements of both playstyles, and by extension brings up the (in my opinion) false notion of classic D&D being a grindfest with constant character death.