Neither do I, and this is a completely irrelevant statement as it doesn't impact my point at all. How often a monster/trap appears in the MM isn't the point because that has zero impact on how often it appears in game play. However, how often these challenges are faced in gameplay does matter. And when in nearly every case, the 5e equivalent is extremely neutered compared to AD&D, that's relevant to how people play and plan in the game. Dismissing that based on how often it appears in the manual is disingenuous.
It absolutely is a mechanics issue, when the mechanics are vastly different for the same scenarios. When the effects of failing a save in 5e is exponentially less than the effects of failing a save in AD&D, that's 100% a mechanics issue. It's not just a one off save scenario, but in nearly every single like v like comparison. Things like level drain are removed completely. As are instant petrification/paralyzation effects from basiliks and medusa (and others). The effects of poison is extremely reduced. In 5e, you get to keep trying to make a save every turn to stop a condition that didn't exist in 1e.
So yeah, mechanics are very much the issue. And most is important, if most of the actual scenarios faced in the game are dramatically different because game play experience is literally based off of what is going in in the game.
Here is the thing, you lead off with difference in play... but since monsters encounter in play are up to the GM, those are GM choice issue not systems.
Whrn you say this "Most undead and venomous creatures/traps in AD&D had a major impact if you failed. Most undead venomous creatures/traps in 5e don't do much if you fail except maybe take some extra damage. " you seem to be referring to system, not some "gm choice" curated occurance.
As I said in response overall, what you encounter "in a campaign" is up to the GM choices and to some degree players choices. The system does not decide to " most " of the threats you actual face are of this type or that. Its not "mechsnics" that decides how many times you meet sacks of hit points or threats you need to prepare for.
"How often these challenges are fcced in gamrplay... " absolutely matters but that is a GM choice, not a systematic one, not mechanics, etc. 5e provides a variety of these the GM can choose from even for GM sticking to the generic statblocks.
"It absolutely is a mechanics issue, when the mechanics are vastly different for the same scenarios."
Now we get to what may be the difference. There is not to me any expectation that I build my 5e expectations on how 1e did things. I dont look for "the same encounter" to produce the same results where "ssme" means using the different statblocks of the same name in the same local conditions.
If I want my 5e encounter to be " blag blag blah" then I will choose creatures within my campaign that produce that, without giving one ehit for how that eoulda might maybe kinda worked in 1e 30 years ago.
So, ser, again, it's about me as GM choosing 5e crestures and encounters that produce the threats, tone, whatever I want. That is not dictated by system or mechanics. I dont have to throw "sacks of hit point" creatures if I want a more "critical threat need counter" type scene.
What the campaign gameplay shows in play due to the threats and creatures chosen and those are not dictated to you as the GM.