Ok, but look at what you need to do to get that old school feel. First, you might need to go back to rolling hit points for players and monsters. The current hit point progression as levels rise gives a very generous "average result rounded up", which I feel gives characters of all levels a good, padded hit point total that they can rely on.
Then you might need to cut this hit point progression down to a slow crawl after level 10 or so.
From here, you might need to remove healing
word from the game, and change cure wounds
so that it can't be upcast, instead bringing back cure serious wounds
as a 4th level spell, cure critical wounds
as a 5th, and heal
as a 6th level spell. Oh and also perhaps remove breath of life
Bards may have to lose access to healing spells entirely.
From there, you might replace death saving throws with "death at 0", 2e "death's door" that lets you linger to -10, losing a hit point each round, or even 1e's original version, which as I recall, lets you linger to -3. And removing "heal from 0" from the equation.
Now if that's not enough for you, perhaps add a critical hit table for more fun, such as this gem from Dragon #39.
View attachment 253394
At this point, you may find your game sufficiently resembles the gritty risk of previous editions to ensure challenging play with cautious, fearful adventurers. If this does not suffice, however, perhaps you might have a copy (or precure one) of Grimtooth's Traps, an excellent series of tomes designed to make D&D (or any game, really) into an exercise truly worthy of "real gamers".