The design philosophy of early D&D is to create plausible scenarios and let players figure out what to do, having the world around them react realistically to their actions. Modules are full of rooms where if an appropriately leveled party ends with swords drawn and starts slaughtering mooks, they're just all going to die, full stop. There's an area with something like 30 orcs in Keep on the Borderlands, which is a module for levels 1-3. Even discounting the bullshit* that nobody misses, the reason people died all the time is that being stupid was supposed to get you killed, not loss of 2d6 hp.
For example, in Keep on the Borderlands, you're supposed to do a lot of scouting and investigating before figuring out how to weaken the various factions in the Caves of Chaos before going in. This isn't spelled out for you. There's no ability check you make to cause it to happen. What you're supposed to is figure out, using your actual brain, that you can't possibly kill dozens upon dozens of monsters with you THAC0 of 19, and start devising ways to achieve your goal without relying on lucky attack rolls. All the old classics are like that...Against the Giants, Temple of Elemental Evil, Against the Slave Lords, etc. There are things that will turn players into paste if they aren't smart, doesn't matter what their stats are.
5e, contrast, has explicit guidelines to make "encounters" that gradually whittle down player resources, but no one of which is any particular danger. Everything is an ability check, and failed ability checks result in harm, not death. All officially published material follows this philosophy, which is why I largely don't bother with it.
If you write an AD&D adventure according to 5e philosophy, modulo bullshit*, it won't be any more deadly than a 5e adventure is. If you design a 5e adventure according to AD&D philosophy, you will put lots of 1st through 5th level characters in graves; getting to 8th level is a sign that you finally figured the adventure out. Mainly, death should feel fair. Players should blame themselves or each other, not the DM, and not rules bullshit, for dying. Ironically, not only do I not find it difficult to do this in 5e, but thanks to how spellcasting has been nerfed into the ground, it's much easier to challenge high-level players than it used to be.
*AD&D's deadliness is half because of its bullshit. Dying because the encounter table put a giant scorpion in your way that won its initiative isn't fun. Having a level 4 character who dies in 1 hit because you rolled 1 hp at every level isn't fun, either. Take out all this unfun bullshit, and how deadly the game is comes entirely down to how you design and run adventures, and how smart the players are, which IMO is how it should be.
