According to Crawford on Twitter, Earthbind causes the effected creature to descend not fall:
Q: do flying creatures affected by Earthbind (Elemental Evil) take falling damage if they reach the ground during the spell?
A: The earthbind spell doesn't cause the target to fall. Probably too subtle: that's why the spell uses "descends" instead of "falls."
Given that, if I cast Earthbind on myself whilst I'm falling, could it be used as a Feather Fall type of effect?
*Throughout, "you" is assumed to be the creature that is falling attempting to cast the spell on themselves
The rule for falling assumes that a creature immediately drops the entire distance when it falls.
When you start falling, you drop immediately with no time to take an action. That is what makes feather fall so useful, because you can cast it as a reaction in response to falling which does interrupt the fall. Earthbind takes an action to cast and thus cannot be cast while falling.
The only way to get around this would have been to have prepared an action to cast earthbind if you start falling next round.
Xanathars Guide to Everything provides an optional rule that allows for non-instant falling.
When you fall from a great height, you instantly descend up to 500 feet. If you’re still falling on your next turn, you descend up to 500 feet at the end of that turn.
So, if your DM is using the rule and if you fall from >500 feet you would be able to cast earthbind.
If you want to other Q&As dealing with casting while falling look at the answers for Does casting Fly on a creature who is falling cause fall damage? and Can you cast a spell with a Doss lute while falling?
The target must succeed on a Strength saving throw or its flying speed (if any) is reduced to 0 feet for the spell’s duration.
Jeremy Crawford says you cannot fail a save intentionally:
No rule lets you opt to fail a save. As DM, I might allow it, assuming you aren't incapacitated or dominated.
So, assuming your DM is abiding by this, you must roll and actually fail a strength save.
If your DM allows failing a save intentionally then this is not an issue.
An airborne creature affected by this spell safely descends at 60 feet per round until it reaches the ground or the spell ends.
If you are higher than 600 feet when earthbind is cast then the spell will run out before you reach the ground and you could take damage.
The description of Earthbind states:
Choose one creature you can see within range. Yellow strips of magical energy loop around the creature. The target must succeed on a Strength saving throw or its flying speed (if any) is reduced to 0 feet for the spell’s duration. An airborne creature affected by this spell safely descends at 60 feet per round until it reaches the ground or the spell ends.
The intent of the spell is clear: it is meant to "ground" flying creatures, but not act as a source of damage itself - hence the specification that airborne creatures safely descend, thus avoiding fall damage. (The word "safely" was added in an errata for Princes of the Apocalypse, and the EEPC was updated accordingly.)
Ultimately, the only potential wiggle room with this interpretation is the mention of "airborne". As far as I can tell, the word is not given any special meaning in D&D. As such, I assume the term is intended to refer to flying creatures - but based solely on the rules as written, I think it could function similarly to Feather Fall:
Choose up to five falling creatures within range. A falling creature's rate of descent slows to 60 feet per round until the spell ends. If the creature lands before the spell ends, it takes no falling damage and can land on its feet, and the spell ends for that creature.
Earthbind is a 2nd-level spell, whereas Feather Fall is a 1st-level spell. Feather Fall is a Bard, Sorcerer, and Wizard spell; Earthbind is a Druid, Sorcerer, Warlock, and Wizard spell.
And perhaps most importantly, Feather Fall has a casting time of "1 reaction, when you or a creature within 60 feet of you falls" whereas Earthbind has a casting time of "1 action"... So to use the latter like Feather Fall, you'd have to ready it with a trigger that occurs before the creature falls - or they'd have to be falling more than 500 feet (if following the "500 feet per round" falling rate in Xanathar's Guide to Everything) and be in the air within range of the spell on your turn.
