I'm asking out of curiosity, because you've raised this point before and it doesn't tally with my experience. Other than conversions of older content, what packs are there that artificially restrict themselves to the classic 8 skills? I know of one (Lemmings Migration), which is hardly evidence that there is a general trend. Are there others?
Well, to be fair, since I announced I would give those older packs an obvious "free pass", I think most of them would qualify for that because they are indeed that old. For example, I know that Nepster's "Return of the Tribes" is even older than NepsterLems, so obviously it only has classic skills, too - even though a pack alluding to Lemmings 2: The Tribes feels much more like it "needs" NeoLemmix skills compared to e.g. Lemmings Migration.

I looked back at when the Lemmings Plus Series started using NeoLemmix skills, and as far as I can tell, it wasn't until Lemmings Plus IV. Of course, I wasn't around at the time those packs were created, so I don't know whether namida made them in Lemmix first, or whether he had already set up NeoLemmix but hadn't introduced any new skills yet.
For packs converted from SuperLemmini, like SubLems, it's also obvious that they don't have NeoLemmix skills. And for the couple of levels Arty had to swap out because of certain skill behaviours that exist in SuperLemmini but not in NeoLemmix (like "Lemmings can't jump"), it wouldn't have made sense to introduce new skills just for those. Just like I didn't want to replace my radiation/slowfreeze levels in Lemmings World Tour with Shimmier levels in New Formats.
Then there is also Lemmings Stampede, although again, I don't quite know when it was first created.
Just in general, when I joined the forums, it already had had NeoLemmix skills for a while, yet most of the "current" content at that time (i.e. the packs held in high regard and recommended to others) still only featured the classic skills. Lemmings Migration came out new at that time and went into the same direction. I think it wasn't until I played CasuaLemmings that I encountered a new pack making excessive use of the new skills. And it wasn't until SEB Lems that a really challenging pack with new skills came along. At least that's how I remember it.
Now in New Formats, there are fortunately already many more packs that use NeoLemmix skills.

However, in my post I was specifically referring to X-of-everything levels. And so far, most packs I've seen that feature them do so in a "X of every classic skill" manner. This is true for Lemmings Plus I-III and NepsterLems above all else. I haven't seen a lot of X-of-everything levels than include NeoLemmix skills in the same quantity as X.
Maybe because we haven't even agreed yet in the first place on what "everything" means when more than 8 skills are available

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I'm referring to both the increased panel size (from 8 to 10) and the increased number of skill types (19 as of now, hopefully 23 in the end) here.