If I were to add them in right now, they'd hate it. Most of us didn't use things like level drain back when they were in the rules. Which is why I would never do that to them, even if I personally liked level drain. In my group, we're perfectly adept at challenging the table, including high lethality, without using level drain (et al).
However, if for whatever reason they were waxing nostolgic and talking about the "good old days of level drain, when things were actually a challenge", they'd laud me for bringing it back.
I mean, we've literally had post-game discussions where the players offered feedback like, "the fights are getting a little too easy, can you up the challenge?"
You have different players, and a different dynamic, than I.
And shockingly, the players didn't complain (except in jest) when the challenge was increased, despite that characters were much more likely to die as a result.
RPGs are a group activity. I'm literally scratching my head at how you imply your players behave. You kind of portray them as a bunch of children who only ever want to win even though that would ruin the game for them, and you the stern parent who forces unpopular challenges on them for their own good. IDK, maybe I've gotten the wrong impression? It just seems like an unhealthy dynamic, if I am in the right ballpark.
We agree that RPGs are a group activity; that's a start.

That said, hockey is also a group activity; where two teams are trying to win and in doing so will often push the rules envelope as far as they can until-unless the ref calls a penalty.
As a player at an RPG table, I see my main purposes being to roleplay my character in-game and advocate for my character out-of-game. Among other things, that out-of-game advocacy includes finding ways to make things easier on my character in the game...which in turn means pushing against the rules envelope if the opportunity arises.
It's the DM's job to push back, and make sure I and-or the other players don't break the game; and it's on me to accept that pushback if-when it comes.
As I've said before, in the case of a new DM it's better to have the system default to easier encounters. That way they don't have to worry about accidentally slaughtering the party while learning the game, and can focus on having the most fun possible.
I get this sentiment. My concern with defaulting to easier encounters is that doing so will set expectations among said new DM's new players that that's how it'll always be, leading to dissatisfaction and complaints when the difficulty gets ramped up later.
It comes back to the premise that "start hard then ease off" is a better formula than "start easy then (try to) add difficulty".
Eleven "bad things" over seven adventures. Six of which were deaths, so that's 5 "perma bad" things over seven adventures. I presume the level drains and limb loss each took place during their same adventures, so that's 3 instances of "perma bad" challenges over seven adventures.
Therefore, clearly you don't NEED these mechanics to challenge your players, if these only featured in something like three out of seven adventures. You might like the variety that they bring to the table, but either more than 50% of your adventures are without challenge or you're able to challenge the party just fine without level drain (et al).
Here's the Bad Things breakdown, by adventure number. It seems I'd forgotten a few...
1. none
2. 1 level drain, 2 major agings (which I'd forgotten about in my previous posts)
3. 1 level drain, 2 limb losses
4. 2 deaths (1*), 1 perma-poly*
5. 1 death
6. 2 deaths
7. 3 deaths, 1 major item loss*
The ones indicated with '*' are either permanent or will require Something Very Big to undo e.g. a wish. The two deaths in adventure 4 were the same character; bad luck for her, and she declined revival the second time. The death in adventure 5 had nothing to do with the adventure itself; the party found unrelated trouble while travelling and it went wrong.
This run covers about fifteen real-world months of regular weekly play.
Again, I'm not arguing that there's anything wrong with using those mechanics if your group is on board. I'm simply refuting the concept that a group cannot be challenged without resorting to them.
Fair enough; and it's not like this lot hasn't had (and still has!) story-based challenges as well. Adventures 5-6-7 above are the first bits of what I hope will be a 5 or 6-adventure arc (or mini-AP, if you like); and sooner or later it'll become clear to them that to complete it they're going to have to find a way to fix that "major item loss" noted above...