Let me be very clear - I'm not very strongly on the side of making this work again. And I want to stress this: Please do not create packs that "require V12.8 exactly". V12.8 isn't a "notable" version in the same way V1.43 (last stable version before some major culls) or V10.13.18 (last old-formats version) is; making packs that will in the future require people to dig it up is very, very messy; and it's very arguable you'd be doing so not because of a culled feature, but because V12.8 has a bug that later versions don't (see below for the logic behind this claim), which is extremely contrary to NL's philosophy as bugs shouldn't exist, let alone be intentionally used by content. Affected levels can very likely be adjusted to use jumpers instead, or some other workaround. (It's different if you put a "this isn't yet tested for V12.9" note on an existing pack; I'm talking about new releases here, especially in the more distant future as opposed to something you've just finished and were ready to release but got hit by this issue.)
As background - there was a bug in older versions where the timing of the faller -> glider transition would depend on what the lemming was doing before becoming a faller. This is now fixed in V12.9 to be consistent, but one side effect is that the Stacker-Shimmier-Glider trick (where you assign a shimmier to a glider right in front of a stacker's stack, so they jump and land on top of it) no longer works.
There are basically three ways this can be handled. In order from my strongest preference to least preferred.
a) Accept this. The logic here - while the trick itself might seem fine, and indeed was accepted by some users who generally speaking have very low tolerance for weird or glitchy physics cases, it only worked due to an underlying bug. Therefore, it would make sense that, like any physics bug, it should be fixed. However, a counter-argument is that at this stage, NL is accepting some very minor ones (such as the floater equivalent of this very bug, which rather than actually seperating lemmings, might just cause a difference of 1 frame in how quickly they hit the ground).
b) Introduce a special edge case where a faller who transitions from a reacher, pulls out the glider slightly earlier than he normally would. This has minimal side effects but does mean there's another special case, just to preserve a behaviour that originated as a bug. Still, perhaps there's some argument as to why this makes sense (that doesn't rely solely on "we could do it before, so we want to still do it now").
c) Revert the fix, and accept the pre-bug situation. The original bug, while very specific, lends itself to creating very trollish levels once a designer is aware of it. It could also in less-dodgy situations, lead to weird unexpected outcomes in challenge runs or when trying things on particularly tricky levels. Not to mention, I kind of feel that "side effects are too much" is the kind of thing that doesn't get decided retroactively; if it wasn't so bad that people were complaining during the RC phase (during which the only complaints related to window sizing / positioning), it's not bad enough to warrant reverting a bugfix. So, option C is pretty much "theoretically, this could be done, but it won't happen".