I can basically agree with Icho and Proxima here, and I think you might have got it the wrong way round: It doesn't seem to me like people reject custom content, it's just that there is less new content available now that has a difficulty comparable to the major packs we used to have in Old Formats. For example, I stopped checking in with all the updates to Icho's Lemmings United, because I know I'm simply not skilled enough for that pack yet anyway. A pack that I would have predicted I might have a chance at is ArtLems, but that one has been cancelled.
For SEB Lems, I got up to the beginning of the Murder rank and simply haven't made any progress since.
Whether I stick to a pack or not has less to do with what tilesets are being used and more with simply how quickly a particular pack starts annoying me. Look to the "Immediate turn-offs" thread to see each individual user's list of personal "no-gos".

I admit I like the original tilesets, but that's mainly about me trying to pinpoint what exactly creates a "Lemmings feeling" in a level (vs. e.g. a "Sonic feeling") in a level. As such, I have tried to create custom graphic sets in the same vein, and I'm also a fan of custom tilesets that preserve this feeling - I'd consider namida's original Tree and Purpe tilesets as good examples here.
In contrast, tilesets like GigaLem's Freedom Planet or your Sonic tilesets feature much greater detail and are arguably of a much higher quality - it's just that I'm somewhat "intimidated" by that, unless I really want to create an artsy level that looks "realistic" in depicting a real location, for example. When it comes to abstract level shapes, I tend to stick to classic tilesets, and since abstract shapes often result in better puzzles since they're less restricted in what they look like than "picturesque levels", that's what I'm shooting for with Lemmings Open Air.
But, as Proxima said, this is about my style of level creation, not about level playing!

That's really just a question of the balance between reward and annoyance caused by a particular set of levels.
Speaking of annoyance: I admit I wasn't sure for a while whether you had developed a personal gripe against
me, or at least my packs, judging from your LP of Lemmicks leading up to that of LWT, in the beginning of which you mentioned right away that you had taken "a step back from my packs" for a while; at that time, I thought it was due to me not using New Formats yet, even though that obviously wouldn't have been possible for Lemmicks in the first place. And while I appreciate you having given my packs another try by LPing Lemmings World Tour, it seemed to me while watching like that pack caused you even more frustration, and not in the "good way" that packs are supposed to frustrate players to some extent

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When I subsequently started fixing backroutes and annoying or misleading parts in the pack, you complained to me about me breaking your previous solutions - apparently the inverse of what IchoTolot experienced when he backrouted your levels.
I definitely can tell you that I didn't develop any personal gripe against you as a result

- but it may have lead to what you described as people just outright ignoring your packs, possibly simply because they wanted to avoid further conflict with you and thus also spare you the frustration of having your levels broken. I can only speak for myself here, of course.
Since you said you've had similar experiences on other forums... yes, maybe there might be a constant in your behaviour that has caused you to experience this repeatedly. But if that is the case, that also means it's probably difficult to change, and I'm not the one to tell you that's what you should do. I just want to make you aware of the consequences that can arise from on the one hand providing harsh criticism of other people's packs, but then responding angrily to other people doing the same with your packs.
I do want to stress though that I only pieced together this picture now, based on what IchoTolot has said about your responses to his replays; I don't remember you ever responding negatively to any of my replays of your pack. That said, I also didn't play as much of it as others did, simply because I've been lacking the necessary skill so far

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Anyways, I can't remember us ever having had any arguments about the direction NeoLemmix should go in, like the heated discussions about culls I've had with various other forum members. And since we haven't grown to dislike each other as a result of those discussions, why should I dislike you then?

It might just be that sometimes, as you yourself acknowledged, Lemmings as a game can bring out certain aspects of us we're not particularly fond of, and I'm sure everyone here can understand that, because we're all familiar with it. We all raged at those stupid little green-heads not behaving the way we wanted them to back when we were kids, and we probably still occasionally do so to this day.
It's just a lot more "community-friendly" to "swear at the Lemmings" together, rather than taking out our frustration on the content creator who designed whatever pack we're currently playing.

The Lemmings and their stupidity should be the "common enemy" that unites the forum, not player vs. level designer.