Let's take a look at their comments. Let me know if I've missed any.
Definitely don't remove secret levels! I find that concept a lot of fun, much like challenges. It’s an extra aspect of the game that isn’t required to solve the game but there for players who want further play-ability.
and as for secret level, why not make 'em alternate exits? or Exits of a different color like if i wanted to make a level with a secret level trigger, I put an extra choice to go to the otherside of the level, and solve the level there, honestly Secret could be handled better
So... I don't like secret level triggers, unless it's very clearly stated where you need to go. I do like unlocking secret levels by completing every level in a rank or getting talismans.
So - we have one "I like it" but without much further elaboration; two that say something that could be considered a more-specific variation on "make the trigger visible", one of whom also does say they're in favor of keeping the unlock by talismans (or by completing a rank, which would be a new method that isn't currently supported).
None of them elaborated very much on these points, unlike Proxima, or many of the people arguing against them. I'd also note that of the people arguing for them, to the best of my knowledge only mobius has actually
used them, and even then only in a compilation pack he was coordinating. However - I think this is one case where the
user's view of them will also matter more than the
creator's - if a creator hides a level as secret, they've made the decision that they don't mind that some people may never see or play this level; however as a user, it means there's content hidden from you that, while you may be able to solve, you may never even know is there, let alone see it, unless you stumble on the unlock method. This is of course mitigated by that, especially in the case of talisman-activated levels, it can be pointed out that they exist.
It's a bit hard to get an experience-based opinion here, as the majority of cases where level packs have included secret levels, it has been via secret level triggers. I think Revenge of the Lemmings might've had a few that were done via talisman? The NeoLemmix Introduction Pack only unlocks them via talisman - although there is a tutorial level for the secret level trigger, it instead takes you to the next
regular level when you find it. Those two aside, the only case I can think of where one was unlocked by a method other than a secret level trigger was one level in Lemmings Plus III Bonus Pack, which was unlocked by finding the code (parts of which were written in various regular levels) and entering it as a level password.
Much of this would be compounded even further with a change to "always unlock all levels" (which is not yet a confirmed change, by the way) and a tree-based level selection. Though the possibility is there to list the level, but mark it as "locked" until the player manages to unlock it. This way, at least, they know it exists, and perhaps some kind of hint can be given as to how to unlock it. I still really am leaning more towards removing the secret level feature altogether, though.