It's too late for the suggestion since it's a physics change anyway, but for the record I don't agree with the notion that it would be unfixable - they can walk and exit while the countdown is inactive, too. Other than cutting off new lemming spawns, the nuke button changes nothing until the explosions start; just because you remove the 5 second countdown doesn't mean you'd also have to also remove the sequential detonation feature that currently exists, so for the most part only levels that rely on the hatch spawns immediately ceasing when the button is pressed would be affected, and that's something that should be fixable by adjusting save requirements. The only type of level I could imagine being unfixable would be levels that rely on it being impossible for the explosions to start within the first five seconds.
Basically, to clarify, by an instant nuke, I basically just mean that the 5 second countdown would be skipped (like, actually skipped, not the fake skip the timer bypass does), but other than that it would behave the same as it currently does - not that all the lemmings would simultaneously explode (at which point you may as well just cull the nuke completely, because it would be a glorified quit the level button). Currently, the lemmings don't all receive their 5 second timers simultaneously - they receive them one at a time (I believe one lemming gets a timer per frame, based on the spawn order?). It would hypothetically still work like that, except instead of receiving a 5 second timer they just explode. Looking through the thread the only real difference that I saw brought up was using a cloner within the 5 seconds. There was also a claim that any level requiring a skill assignment during the countdown would be broken, but I don't buy this claim for skills other than cloners: what's special about the countdown status specifically, rather than the fact that it's just time before the explosion occurs; i.e., why couldn't these assignments be made before the nuke is triggered? There was also disagreement regarding some of the examples of broken levels that was never noticed/addressed -- Icho claimed Supernova as an example of a level that would be broken by the change even if the sequential detonation was kept; Crane claimed it would still work as long as the sequential detonation was retained.