Deliberate exclusion. I was listing the things I would permit.
However, if it was Darkness and Silence, then I would let him cast it when he left the Silence.
In other words, the spell is designed to let you react to falling by letting you fall slowly. As long as you do it as soon as you are able, and not try to game the system to get extra advantage, then I'm not going to prevent it from working.
Oh, and regarding the "splat" rule: as I suggested earlier, I do agree upon further reflection that death is maybe too harsh a penalty. I think I would prefer an option where failing the acrobatics check results in a penalty that isn't death. Maybe stunned for a round, and starting the subsequent round prone.
Or something to that effect.
See, I do not know about your players, but if i told them "hey, whether or not you can cast FF as a reaction when falling out of a silent-darkness will be determined by whether or not you could have cast it before and if i think you were trying to gain an advantage - i am pretty sure my players would check me for drugs... not necessarily to stop me from taking them as deciding they needed them to keep playing in my game.
I would feel stupid explaining that the "can this be done" would be dependent on the intent as opposed to the actions, approach and circumstance and that would violate my STUPID RULE which says that if i would feel stupid explaining a rule to my players, i dont use that rule.
i would also not be inclined to try and cover that as an interpretation of RAW based on the definition of "is",... wait.. of "falls" when my actual "reason" is derived from my perception of their "try to game the system to get extra advantage"
Matter of fact, if i just read FALLS as FALLS and not FALLS BUT ONLY FALLING IF... then choosing when to trigger feather fall is no more a "try to game the system to get extra advantage" than is moving to a position before casting Thunderwave so that it knocks a creature over a ledge.
case 1 - My bard can move to the left side or the right side of the creature with the same movement used for either.
if i go leftside i am between the creature and the drop and my thunderwave pushes them further into the room leaving me at the ledge where i can be shoved off by any number of possible situations.
if i move to the right side, i am in the room without a dropoff worry and a thunderwave failed save pushes them off the ledge giving me a big gain.
As long as the Gm does not change how things work, i get a lot more from making one decision and suffer a lot less risk from the other.
this is NOT to me a case of "try to game the system to get extra advantage" but is a case of playing the character as intelligent and using the way things work in game competently. The advantage gained is not "extra" its just what one should expect from making the better decision.
Similarly...
case 2 My character an pushed off a ledge by some meddling bard's thunderwave for a long long fall.
I can choose to cast FF now and slow my fall right here... possibly getting attacked by those other meddlesome kids, possibly having the FF run out before i hit bottom, possibly getting attacked along the way by flying beasties or at least fireballs since 60' per leaves me in range
i can fall for a while then cast my FF when i get close, getting me pretty quickly out of the range opf most of their spells/attacks etc.
As long as the Gm does not change how things work, i can get a lot more from one choice than the other.
this is NOT to me a case of "try to game the system to get extra advantage" but is a case of playing the character as intelligent and using the way things work in game competently. The advantage gained is not "extra" its just what one should expect from making the better decision.
But simply put, whether or not i thought it was an attempt to game the system or not would not affect or impact my ruling from one case to the next. My ruling is about the "way things work in the setting and the game" not "is this a try to game the system to get extra advantage"
but thats me. others may disagree and that is fine.