as we all can agree that 3e, 4e, and 5e rules are not as lethal as previous editions (removal of save or die, increasing ability scores, powers gained at almost every level, assumption of increased magic items, etc)
Yeah, I've never known us all to agree.
3e, for instance, was plenty deadly, it went all-in on giving monsters the same options as PCs, so much of the assumed advantages the system quietly gave PCs in prior editions quietly vanished - also 3e retained SoDs, /and/ saves didn't keep up with DCs, in contrast to prior eds where saves genuinely improved with level.
However, when looking at the rules, it seems 2e just might actually be the most lethal edition.
I'd tend to agree. 2e really didn't change the PC side of the equation too much from late 1e. But, it really goosed a lot of monsters, giving them more hps, bigger damage, and the like. I'm not sure, but I think save penalties may have become more common, too.
So...when ranking the editions by lethality, it goes 2e>B/X>OD&D>1e>3e>5e>4e
Hmmm....discussion?
IDK about concluding with a detailed ranking including things you excluded up front. But I certainly see the case for 2e. The case for 3e is also strong: monsters were very deadly, with piles of hps, loads of damage, SoDs, massive STR bonuses to hit, etc - OTOH, /optimized/ PCs were utterly horrific.
And, I think that ranking holds up best a 1st level, when PCs had few spells or other special resources to draw upon, and the lowest hps of their career, across the board. In general, as the eds progressed, 1st level PCs were made more durable (peaking in 4e, though - 5e finally reversed that trend). I suppose that contributes to the earliest eds are most often held up as the deadliest. But, while 2e didn't much buck the trend in terms of starting PCs, it did start a trend of making monsters dramatically bigger & badder.
At higher levels it felt like the older eds really dropped of dramatically in lethality, the same is true of 5e, now, but just out of apprentice Tier lethality drops off. In 3e & 4e, lethality changed less with level, I think - certainly in 4e it stayed pretty stable, in 3e it changed more with system mastery than anything else.
(Yes, I know any edition can be lethal depending on the DM, but this is factoring RAW, all else being equal).
Another thing to consider is how the system facilitates the DM pegging the campaign to his desired level of lethality. TSR eds offered virtually no tools for that, over the years, most of us developed good instincts for it, and we could always take enough of combat resolution behind the screen to adjust an encounter as needed on the fly (if didn't offend your sensibilities). 3e & 5e use CR, which is not at all dependable, and 4e, EL, which was more intuitive & gave more consistent results. But, 3e & 4e were also more likely known quantities to the players, so if you did screw up, your latitude to adjust was limited - that is, it's just as well 4e gave you predictable results, because once you laid out the monsters and displayed their powers, you couldn't readily soft-ball it half way through, it tended to be all above board. the 'fudging' would be very obvious; similarly, the monsters' options were generally known to the players (even if you kept the stats behind the screen, they quickly infer a lot about it), so it's abilities 'changing' at some point in the combat would likely be noticed.
As seems to be usual, 5e found a solid compromise: it's monsters do more damage (and the party has more healing), and while it's CR formulae or complicated and BA makes outnumbering the party very dangerous, the Empowered DM can take much of that behind the screen, and use 'rulings' and narration to salvage an encounter that turns out to be much easier or more deadly than intended...
...though, I acknowledge (before someone points it out), run 'straight,' especially if player options are turned on, and/or magic items in play, it breaks "easy" quite readily.
