I think the point I agree with
@Snarf with the most is that it is INCREDIBLY dependent on the table. For example, he talks about adventures being full of deadly traps. Thing is, the modules generally actually weren't. Yup, there were a few. But, by and large, they really weren't.
Additionally, just as a point of order, I don't think anyone would argue that 5e is more lethal than 1e. Certainly not me. For my money, 3e was by far the deadliest edition. It was just so easy to kill PC's. When you have orcs that can quite possibly do 25-35 points of damage in a single hit (Greataxe was a x3 crit weapon, dealt d12 and orcs in 3e have an 18 Str - max damage of 36 points with a lucky hit - enough to straight up kill 3rd level PC's.
Which isn't possible in AD&D. Nothing deals that kind of damage. An ogre, from memory, dealt like a d8 points of damage per round. No strength bonus. No bonuses at all. Maybe a d12? As I said, I'm going from memory.
The point that I made was that AD&D COMBAT, not the edition as a whole, is not very lethal at all after about 3rd or 4th level. The baddies just didn't do enough damage, nor did they have any to-hit bonuses. Which meant they weren't hitting often and when they did, they didn't really do that much damage.
Add to that the impact of Unearthed Arcana - which absolutely did change how the game worked - as well as things like Dragonlance, which also played a pretty formative role in my gaming experience, and no, I don't think AD&D combat is all that lethal. Most of the lethality came from stuff that bypassed the combat rules like poisons and the like.