I ran across this old video of Heian Nidan and thought I'd share an observation.
(The channel has old videos of the other Heian katas too.)
Notice the folds of the knifehands. Not counting the first one after the side kick, the next six knifehands are in three pairs. In each case, the first knifehand folds with the secondary hand pointing straight out, the way modern Shotokan does it. But the second knifehand of the pair folds with the secondary hand swept back behind the head. To my knowledge, this style of fold is only used in TKD these days, but it is interesting to see that it was once standard in Shotokan.
My bunkai conjecture is that the first knifehand of each pair is intended defensively. The shuto blocks a wrist or arm, and grabs it. The second knifehand is intended offensively. You step in, retaining control of the secured wrist/arm, but with a kind of "high hikite" you pull that hand back past your head to deliver a neck strike.
For comparison, classic TSD folds a knifehand with both hands to the opposite hip (see here. https://www.youtube.com/watch?v=Ph9ybOTMzbg ) Which I would interpret as a "low hikite" followed by a neck strike.
I don't have much to add beyond the interesting (to me) historical observation. If any Shotokan practitioners have insight as to when or why the official change occurred, I'd be interested in that too.