Yes, you can L-cancel dair, but there's more to it.
You will need to do a no-stall dair (hold down while dairing), then fast fall in order to hit the ground before the dair autocancel window begins. Actually, if you do a frame-perfect dair (initiate the move on the first frame after leaving jumpsquat), then a no-stall dair will NOT autocancel even without the fast fall.
Dair frame data goes like this:
0 input dair
1-7 wind up
8 sweetspot hitbox (bulb)
9-10 hitbox
11 cooldown, FF possible (if no-stall)
12-20 cooldown
21 FF possible (if falling)
2? FF possible (if rising)
Whether Ivysaur stalls or not is determined by the position of the control stick as the game enters the frame of the first hitbox. If you are holding down at all (there does not seem to be a threshold, stick barely down and stick fully down give the same result) as the hitbox begins, Ivysaur will not stall after the dair. Holding down as you dair is OK for a no-stall dair.
The fast fall input seems to be read on or after frame 20 of normal dair. The fast fall input is read on frame 11 during a no-stall dair. Simply holding down as the move executes is not sufficient to fast fall during either dair. You must input a second downward input on the control stick.
I didn't know this:
the timing of the FF input differs whether Ivy is rising or falling. This explains why it seems inconsistent during matches. I was not able to come up with a consistent number for when FF is possible while rising, especially during fullhops. Sometimes it seems I could FF on frame 22, sometimes frame 25, sometimes 26, and sometimes over 30! I recommend that if you are trying to FF dair, do a no-stall dair for its consistent FF window.
Normal SH dair:
Airborne frame 6, hitboxes frames 13-15, FF frame 30, land frame 37 (autocancel),
actionable frame 41
No-stall SH dair, no fast fall:
Airborne frame 6, hitboxes frames 13-15, land frame 31 (L-cancel),
actionable frame 41
No-stall SH dair, fast fall:
Airborne frame 6, hitboxes frames 13-15, FF frame 16, land frame 20 (L-cancel),
actionable frame 31
@
S
Sixonesix
There is a difference between L-cancelled dair (which has 10 frames of lag) and autocancel dair (which has 4 frames). L-cancelled dair, however, gets Ivy back to the ground considerably faster (10 frames). Given our slow character, 10 frames is a godsend when techchasing or comboing. I do agree that mastering uair's L-cancel is much more relevant to the character.
Unrelated- this series of inputs is kind of semantically similar to peach's low FC aerials (jump, down, attack, down).
Shoutouts to insomnia