Okay, so sounds like culling it actually did cross your mind before, you aren't merely involved due to being the one who would end up making the changes.
Do we think a bottom-to-top bash stroke animation can ever look natural? I could be mistaken, and the real world hardly uses Lemmings/Lix physics, but I believe all burrowing animals have evolved to use basically the same sort of top-to-bottom motion as the Lemmings basher does, when going horizontally. I think it's because it works better with gravity and avoids having dirt flying up the air right into your face.
For reference, in Revolution the animation is not even really much of a stroke if I recall. It looks more like the lemming is punching the wall in front, with the bottom block removed first on first punch, then the top block on next punch ("block" because due to practical limitations with 3D graphics, the physics there are based on larger block units rather than individual pixels). So it's more of the "judo punch" approach then the classic bash stroke. I do have to admit that given they could've just as easily remove both blocks at once, the "conspiracy theory" that they purposely removed the bottom block first to avoid staircasing is perhaps far more plausible than the usual conspiracies of the world.
The directionality argument is an interesting point, on the other hand it is at best a minor secondary argument, since there are other examples in the physics of using skills in "atypical" directions: repeat platformer+basher to stack bricks diagonally like a builder bridge, repeated builder+digger/basher allows for an almost-horizontal (certainly less sloped) bridge, you can get through a wall horizontally with enough repeats of dig+build, etc.
How about miners? In both Lemmings 1 and Lix you can also do "miner staircase". Of course it's even worse to execute as you must interrupt on precisely one exact frame. I suppose this at least severely limits its use in multiplayer due to lack of pause/framestepping.
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aside from Revolution, does anyone know/remember/can look up whether other games in the series support or not support this behavior?
I actually had a chance to test stuff out just now. The results are more interesting than expected:
L2: As noted, they left the basher behaviors basically as-is. Though they did shorten the maximum height of a walkable step. But all that means is that you can't make quite as steep a tunnel as you could with L1 physics (and that's not very relevant anyway in multiplayer context when you can't pause or framestep). I do think it implies the corresponding miner staircase does not work due to too high a step, but forgot to check.
It might work with the scooper though? [edit: brain clearly not working, the scooper motion is one of very few that is bottom-to-top]
Club basher turns out to be not very exploitable. Because of the much larger club-swing, at the point where you'd normally interrupt, the step is too high to actually walk up to. Then at the next frame, so much more terrain get removed (practically like 90% of the entire removal chunk) that all you get is a pathetic "step"-height of around 1-2 pixels. So technically it still works but not very effective compared to the regular basher.
The fencer does indeed take the judo-punch approach, despite all that fancy waving of the sword's tip in the animation disguising it. There is no partial removal happening, a discrete chunk is removed upon exactly one of the early frames of the full animation cycle. (Granted, one can argue that since the fencer tunnel naturally goes slightly vertically up as well, making a staircase is somewhat of a moot point; even if it works, it would merely allow you to achieve a steeper slope than the natural one.)
L3: it actually looks like you can't interrupt anything in mid-stroke. It's true that there isn't that much of a stroke to start with, but so far when I try I haven't succeed in stopping anything mid-stroke. The game appears to queue the skill/action assignment and only carry them out after the current cycle of action is completed.
Of course, more to the point (one which I can't believe I forgot), the L3 skill system actually combines basher/digger/miner by actually allowing you to pick the direction of the tunnel and even change it on the fly. And diagonally-up is a valid choice for the direction, so you might as well just do that.
Revolution: the miner staircase sort of works, but can't be easily applied repeatedly. It's hard to depict what happens w/o a picture, but basically you can't be too close to the wall if you want to create a walkable step. As a result, if you want to apply it repeatedly, I think you'd need to spend one more builder per step (so twice the usage of builders overall) to create the necessary room for the miner to start mining not too close to the current end of the tunnel.