All that said, I would say that even when you play within the guidelines, it is still easy mode compared to the older editions, simply because of other rule changes ... for example:
Save or die -> save or suck.
Level loss -> doesn't exist.
System shock + loss of constitution -> doesn't exist.
Death at 0 -> Whack-a-mole.
Need for clerical healing -> Oprah healing (You get healing, and you get healing, and YOU get a free car ...um, healing).
One thing that I've pointed out elsewhere is that it's actually harder to die in AD&D than 5e, although the consequences are quite different.
In AD&D, you didn't die until you reached -10 hp. From the 1e DMG:
"When any creature is brought to 0 hit points (optionally as low as -3 hit points from the same blow which brought the total to 0), it is unconscious. In each of the next succeeding rounds 1 additional (negative) hit point will be lost until -10 is reached and the creature dies. ... It ceases immediately on any round a friendly creature administers aid to the unconscious one."
Note that it's "any" creature and it doesn't require any checks, just providing aid. Where it differs is the consequences:
"Any character brought to 0 (or fewer) hit points and then revived will remain in a coma for 1-6 turns (10-60 minutes)
. Thereafter, he or she must rest for a full week, minimum. ... This is true even if cure spells and/or healing
potions are given to him or her, although if a heal
spell is bestowed the prohibition no longer applied.
If any creature reaches a state of -6 or greater negative hit points before being revived, this could indicate scarring or the loss of some member, if you so choose."
Note that this became an optional rule in 2e, otherwise you died at 0 hp.
And frankly, it's the "get knocked to 0 hit points, then get revived an act like nothing happened" aspect of 5e death and dying that I like the least.
So in AD&D it takes 7-10 rounds to die. In 5e, it can take 3-5 rounds to die.
In AD&D, however, you will die without assistance. In 5e you can spontaneously recover.
In AD&D, you are finished adventuring for a week. Your party has to drag you around and protect you, leave you there and protect you, or everybody returns to town to recover. In 5e, you just continue.
So AD&D
feels more challenging because you have to work hard to make sure nobody is reduced to 0 hit points. That dramatically alters your tactics in combat. But mathematically, it's actually easier.
--
Although I haven't done the math, I suspect that this holds true across combat. For example:
I think that ACs in 5e are generally lower than AD&D.
I think that hit points are generally higher in 5e than AD&D.
But I think it might take roughly the same amount of rounds to win a combat.
That is, I think that it takes fewer hits to kill a monster in AD&D, but you hit less frequently. So the number of rounds is probably similar.
In 5e, you hit more frequently, but it takes more hits to kill the monsters (not including burst damage, etc.).
It could be mathematically identical, but the feel is different between hitting more than less frequently.
Combine this with the fact that in AD&D you're trying to avoid being reduced to 0 hp, rather than the often encouraged "it's most advantageous to wait until 0 hp to apply healing), I think that it reinforces the idea that AD&D is more challenging.
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Aside from the lack of save or die, and things like level drain, the one thing that I think does make 5e easier is the frequency and amount of healing. This is particularly evident in something like Tomb of Horrors, since each day you start with full hit points in 5e. This more than anything has greatly altered how older adventures play in 5e. In fact, I should complete an extensive study I started as to how deadly ToH is in AD&D (it's not as deadly as you think). But it's virtually impossible to have the same experience in ToH in 5e.
Overall, I think AD&D is actually "easier" in most regards when looking at the math, but the feel is much different. The slow healing combined with "don't get dropped to 0 hit points" is what gives it a much grittier feel, even if the math itself doesn't bear that out. And I admittedly haven't verified all the math yet.
In the end, it's probably the feel that matters more than anything. And I think this is a good example of the feel that I think 5e is lacking for our game (and why we've made so many changes).
