Yes with a small caveat (3d6 in order, random HP at L1 instead of maximum and save or die stuff ) its a myth. In fact the opposite is true
Lets use B/X (Lab Lord) Fighter vs Orc as a demo. Note that i am using 3e style AC for simplicity
L1 BX Fighter Gets +1 to hit and damage (from STR) 9 HP (from Con) has AC +6 (mail and shield)
his Orc (as per LL) is AC+4 HP5 and does 1d8 with a to hit bonus of zero
This means our fighter can survive 1 or two strokes from the orc and baring bad luck, will be able to drop his orc, probably in one blow
Now of PF Fighter
He is +4to hit, +2 on damage (from feat, level and STR) has 12 HP with Con and AC +8 (scale, large shield, dex +1)
his Orc (as per SRD) is +3 to AC, 6 HP +5 to hit and does 2d4+4 --
This means while the chance of the fighter hitting and dropping the Orc are higher, the reverse is also true . Its easier for the orc to hit the fighter
Also while 1 good roll (either a good die roll, or a crit) can drop our PF fighter , the B/X fighter can always survive one good shot.
And yes this extra lethality pretty much carries over to all the low level monsters.
The only real mitigation in Pathfinder is the "die at minus 10 and stabilize rules" and of course the presence of more magical healing, the 1st of which is an old school rule and the second can be added via healing potions.
Now if anyone is wondering where that myth probably originated. I'd guess three things
#1 There was less effort at scaling encounters. Often as not no one cared, if you were dumb enough to mess with something bigger than you and couldn't get away, well ya died.
#2 Many of the old classic modules, even such things as the Keep on the Borderlands were Meat Grinders
#3 The hobby was pretty new and many DM's were unskilled and as such did not know how to DM in a way to challenge rather than wipe out players with bad luck.
However, myth or no myth, with a few key exceptions, older editions were less lethal mechanically.
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