Over on Back to the Dungeon, Eldrad wrote about old-school gaming style with the newest of the D&D rules, 5th edition.
Old School 5E Without Changing the Rules
I would sum that up as, it's not as much the the rules, as it is how you play them.
My own GURPS game is an example of this.
It's hard to die in GURPS, even though combat is potentially lethal.
It's easy to spam out magic and rest and recover from injury - so much so that the rules have their own special cases to limit that.
It's a skill-based system with non-random character generation, something that dates back to prior to much of the old school games but yet is mostly associated with newer school games.
Given all that, you should really need to house rule the hell out of GURPS to make it play lethally, right?
Not really. The campaign graveyard is full to bursting.
The PCs risk death every delve.
They can't rely on rests any more than Eldrad's PCs can rely on a short rest when it's convenient.
Rules have a heavy influence on how the game runs, but you can run a game in a style that changes that.