The lack of easier packs in the new format version is known, but how did it come to this?
I think a good starting point for that would be our discussions about immediate turn-offs (https://www.lemmingsforums.net/index.php?topic=3926.0) when seeing a new level for the first time.
Crowd control as an overarching term is a necessary part of pretty much every level, and I think there are clearly three tiers to it:
Level 1: crowd containment with blockers (--> "Noob strategy" :P )
Level 2: crowd containment with digger pits & Co., usually enforced by requiring to save everyone, or simply not providing any blockers (--> Intermediate)
Level 3: flow control, i.e. no ways to contain the crowd at all, it's all about keeping them busy and timing things just right (--> Professional)
Of course, this doesn't account for difficulty points unrelated to crowd control.
Except that that topic showed that we have a lot of differing opinions about what turns us off from a level. And if we're thinking about new players, how can we know what would turn them off when they are just getting to know the game?
For example: You list "10-of-everything" levels as a massive turn-off; but I think it helps new players to have more sandbox-like levels in which they can play around and find out what the skills do without being in any real danger of losing.
Aside from the fact that some levels don't have a crowd because there are only a small number of lemmings
there's a major consideration you've missed: some levels can have no way to control the crowd, but the save requirement is low enough that you can let the lemmings behind the worker die until the path is created. In GemLems, four of the first five levels are of this type.
I stand by my point that 10-of-everything levels don't teach the player anything.
A crowd to me is any number of lemmings larger than one. In those levels with, say, 1 to 6 lemmings, where you often have to steer every single one of them individually along various paths, I'd regard that as a form of flow control. Characteristic trademark of this: All of them are moving pieces. There is no group of lemmings that you can just leave behind without having to worry about them.
Okay, but then levels with only a small number of lemmings can be very easy even though they would technically fall into your "flow control" classification, e.g. "Nightmare on Lem Street".
I rebutted that point with a detailed examination of what the player learns from each Fun level. That's not just hypothetical; it's how I learned the game. And sure, it didn't put me in a position to cope with really difficult puzzles (even solving "The Steel Mines of Kessel" on Mac took me ten years after beating the rest of the game); but it did equip me with the knowledge I needed for the mid-Tricky levels where it stops holding your hand.
but I would respond that they have to get from A to B first
However, many packs (notably including ONML) skip the part of the game where players are asked to solve simple puzzles. They're mostly treating the x-of-everythings as an obligatory scrap to throw at less experienced players before getting to the actual puzzles their creators wanted to design, rather than using them as a tool to build experience in these players. If a pack is going to skip directly from x-of-everythings to hard puzzles, then it should just skip those levels. Less experienced players would be better served by these packs being upfront about their difficulty.
The main issue with open-ended 10-of-everything levels is that you can try almost anything, but you don't really have to do anything specific. And while it might be fine to have such a "playground" level either at the very beginning or the very end of a rank, so that the player can test out their current amount of knowledge of the skills, every further one of those levels that gets added in succession weakens the purpose.
In short:
Skill-restricted levels are the lessons. That's where you teach the player.
The 10-of-everything level is the schoolyard. ;) That's where you send the player to take a break and toy around.
Strato: So is Dullstar essentially right in saying that your problem is not with the quality of the 10-of-everything levels themselves, but the difficulty jump that follows? If so, then why on earth did you not simply say so? If you'd said that way back in the original thread, we could have had a much more profitable and interesting discussion. But when you say "Custom 10-of-everything levels tend to be Tame-like rather than Fun-like", that sounds like you're saying they have the same flaws as the original Tame levels -- no hazards or risk of failure; extremely simple solutions; no progression within the set.
Another is to do X-of-everything but omit a useful skill, as in "Heaven can wait (we hope)".
Later Lemmings Plus-packs, I think starting around LPIV or so, began with easy puzzles right away. If I remember correctly, namida actually made a survey about precisely that: How the forum members wanted future releases in the Lemmings Plus series to continue and what the early ranks should look like.
I'm just adding here that I have been working on my (old) level pack and I'll probably be releasing it within a few weeks or so (maybe sooner even). And it's by now of easier difficulty overall; than most packs these days. Actually it was a bit on the easy side even for when I first made most of the levels around 2012-2013.
"proper maintenance of content" is something that I won't promise. As to backroutes of my old levels, most of them have been long since fixed to a good point and to any new ones I probably won't care. Beyond that I'm likely not going to update the pack for any breakages due to changing game physics or graphical changes. If anyone wants to take over that job at that time they are welcome to.