The concept is pretty simple but interesting imo.
When a Lemming walks on it; 1 pixel layer of terrain underneath him disappears so the next lemming behind him would fall (or take away another layer
underneath it]
It was used in L3, and Revolution at least. Somewhat effectively I think, albeit very sparsely at least in Revolution.
Some things that should be discussed:
What if a clump of lemmings encounters it?
Should all disappearing terrain look exactly the same? OR be different according to tile set. If according to tile set it should stand out in each tile set as being recognizable.
This is what the majority would think, I don't really care personally. I'm in favor of artistic freedom.
There's less need to limit artistic freedom for clarity, now that the user can just activate Clear Physics Mode if they need more clarity. This is precisely why this was introduced, in fact. (Of course, there are limits - if even someone familiar with your set needs to regularly go out of their way to use it to figure out what's what, you should probably adjust the set.)
With that being said, this is another one of those features that complicates things a lot, both code-wise and game rules-wise. There'll need to be a lot of support for me to consider introducing something like this, and as with the new skill suggestions, most likely some kind of trial period (this still hasn't happened for the new skills, so I can't give any timeframe on when this might occur).
The way I remember it working in L3 is that once at least one lemming has stepped/landed on a disappearing tile, the tile starts to fade out visually, but physically remains solid until completely faded out over a short period of time (about a second I think?). Any additional lemmings that may step onto the tile during its fadeout, or if multiple lemmings stepped onto the tile on the same frame that started the fade-out, have no impact on how fast the fade-out goes.
The disappearing tiles in L3 are small (ie. short), thin horizontal straight pieces. Normal terrain tiles IIRC are all thicker so there's no risk of confusion. In addition to stacking them vertically (and of course putting a bunch together horizontally), you can also create descending and ascending diagonal-ish staircases out of them (the latter because in L3, lemmings are no longer single pixels in their walking physics, and so can step up such steps even though they are not directly connected.
Conceptually I guess it can lead to some interesting puzzles. It basically creates a way to isolate a small number of lemmings away from the rest, in a manner that consumes no skills. Granted, it's possible that something like a limited-use teleporter can more or less do something similar with less implementation complexity.
Some complications of implementing this in NeoLemmix's existing physics is that terrain can overlap each other (they can't in L3), and terrain removal skills in NeoLemmix is pixel-based (they are tiled-based in L3, so AFAIK there is no way to remove portions of a disappearing tile; the whole thing will get removed). That's not to say they can't be handled, just that it's a little more tricky in NeoLemmix. You could also potentially make disappearing tiles act like steel instead when they haven't yet disappear, to potentially sidestep some of these issues.
I dunno, it's an intriguing idea but I feel like there are probably other ideas that are more productive puzzle-wise and also easier to implement. Making them easily recognizable across different tileset styles will likely also be a challenge.