First of all: While I am against reiterating the same arguments over and over after having reached a conclusion, I think it is totally fair game to open up a discussion again, if new developments add substantial arguments to one side or the other.
Arguments for solid ceiling:1) Existing content, that relies on it.
2) L1 had this behavior. As NeoLemmix already changed quite a lot of stuff, I suggest to disregard this point.
3) Climbers may accidentally vanish out of the screen.
4) Sky isn't deadly, cf. Dullstar's argument: Well, if you really want an in-game reason, one could always say lemmings will die due to asphyxiation due to thin air this high.
More problematic is the question: If the sky is considered air, then why do builders turn around when they hit the ceiling, instead of continuing forward?
Arguments against solid ceiling:1) The ceiling is currently an exception: All other sides are air and lemmings die when they reach them. So there is no reason to expect that the ceiling behaves in any way differently.
2) Shimmiers should not be able to shimmy across the top border. With a non-solid ceiling, this is a non-issue. Otherwise it would require ugle special-casing to determine whether the shimmier has actually grabbed terrain or the level ceiling. And this special-casing would even be needed during each frame, because shimmiers may at first hold onto actual terrain and then reach the ceiling while already shimmying.
Moreover it is not clear to me why there should be a difference for shimmiers but not for builders for an empty ceiling compared to one that has a 1-pixel high terrain ceiling.
3) What should we do with jumpers, which will be implemented at some point, too? If we consider the sky to be empty non-shimmyable, then the jumper should be able to move through the ceiling with his head, instead of hitting and dropping down.
Upshot is: With the shimmier and the jumper ceiling interactions become much more frequent and muc more varied. If we keep a solid ceiling, then we will need a lot of special rules to define what happens to shimmiers and jumpers, while the rule-set would be a lot simpler for non-solid ceilings.
Therefore I am acutally much in favor of non-solid ceilings, which will be far easier to learn and understand given the new additions to the skill set.
I made a new-formats version on NeoLemmix with non-solid ceiling for you to try out, cf. attachment. Surprisingly few replays fail actually.
When I played through Lemmings 2, I found the deadly ceiling to be one of the most annoying aspects of that game (granted, the flinging mechanics made that much worse than it would be in NeoLemmix, but still). Considering that the ceiling's behavior doesn't usually result in backroutes, the ability to put builders close the ceiling without having them die randomly because they went slightly too high is rather nice - besides, it's logical that lemmings, if allowed to go offscreen towards the top, would eventually fall back into the level area due to gravity, which isn't the case with the bottom (because you can think of it as a bottomless pit) and the sides (where there's no particular reason why the lemmings should EVER turn around). It also has the convenience thing of not having to babysit climbers as much - and if the level designer really wants to fry rogue climbers it's easy to use a flame trap for that, especially with style mixing.
With the skill blueprints accidentally building out of the ceiling or jumping out of it will be more or less a non-issue. And jumping out of the screen with a shimmier... Let's just say I have faith in my fellow players that they will be able to recognize when there is no solid terrain above the lemming
.
Regarding the climber: Yes this is an issue, but even now I would certainly encourage level designers to put terrain there, if the climber is intended to drop down again and not rely on
the player knowing that the ceiling acts solid.
It is also logical that the top edge of the level represents sky.
Then please explain, why climbers should drop down if they reach the ceiling, if it is so logical for it to represent sky.
You cannot hold onto the sky, therefore shimmiers should not be able to attach to it. To be honest, I don't think it's even all that inconsistent to disallow the shimmier from using the ceilng, because while we sometimes consider the ceiling to be technically equivalent to a mass of steel just offscreen, there's no particular reason it needs to literally behave like a mass of steel at the top of the screen. The main reason I think it makes sense to have lemmings hit their heads on the ceiling is because you have to set an arbitrary "you can't build here" cutoff SOMEWHERE to prevent the lemmings from walking on top of the level boundary - and having them hit their heads certainly makes a lot more sense than them randomly dying because they wandered 1 pixel too high.
I am no really convinced here, that turning around due to illogical steel at the ceiling is in any way better than arbitrarily dying there. Certainly neither is prefect, but I cannot see why turning around is inherently more logical than dying. More convenient perhaps, but that wasn't your argument in this paragraph.
And finally I just have to ask: Why should a builder turn around, when hitting the ceiling
with his head, if there is no steel at the ceiling? Why not continue until the builder's feet hit the ceiling? If the ceiling would be considered sky, then certainly this would be much more logical, wouldn't it?