When the monster list includes but not limited to:
ghouls
ghasts
ghosts
specters
wraiths
whites
vampires
basilisks
beholders
medusa
cockatrice
green slimes
black puddings
rot grubs
rust monsters
poisonous (sic) snakes
spiders
psuedodragons
carrion crawlers
catoblepas
Any poison trap
etc
etc
are all significantly more dangerous in AD&D than 5e, it's disingenuous to argue that the difference in deadliness between editions isn't mechanics, but GM preference. I know you know better. It absolutely is mechanics, not just with the monsters like v like themselves, with with 5e mechanics that allow you overcome conditions with a save attempt every turn.
The only way your argument remotely holds water is if the typical D&D game never encounters any creature or trap that is poisoned, never encounters any undead, and never encounters any other creature in that above list or creature that has magic resistance or weapon immunity that their 5e counterpart does not.
My point being, is that any adventurer worth their salt will plan ahead with the knowledge available to them, and put focus on the higher risk areas. In AD&D, that meant you planned for all of the above or you didn't last long. In 5e, since those risks are all extremely less than they were in AD&D, players don't spend nearly the effort planning for them. And IME, is a reason why every battle begins to feel the same for a lot of people, because they approach every battle very similar---bags of HP and and how much DPR you can do regardless of any special ability because the risk of suffering a bad save is much less than it was in AD&D.
If you're arguing that 5e mechanically is not less deadly than AD&D because it's all up to DM preference, then I'm sure most would laugh at such a statement. Everything being equal, the mechanics of AD&D were more lethal than 5e. It would be like me arguing that 4e was the most deadly edition because one DM made every player only have 1 HP and put them in a meat grinder, while the AD&D DM gave all the players max HP and monty haul magic items, and never had them face anything dangerous. Needless to say, that would be a very flawed analysis to make.