This glitch was discovered by accident at a Premiere Challenge hosted by OriginGamma
If a Pokémon uses Sky Drop on a target that used Spiky Shield, but faints due to Spiky Shield, the user of Sky Drop will have its HP reach 0 but remain on the field. It cannot use moves (creating a dead slot), but it also is considered conscious (making it impossible to move).
Would be interesting to see what happens if both sides only have a Pokémon with 0 HP on their field at the same time...Source and video
Also, it's worth noting this is the third Sky Drop glitch discovered in Sun and Moon. There's the Red Card Mimikyu bug mentioned in this thread
; and there's the this bug mentioned on GameFAQs
that causes the target of Sky Drop to become invisible if the user of Sky Drop faints due to Rocky Helmet/Rough Skin.
I checked both the Spiky Shield Sky Drop glitch and the Rocky Helmet/Rough Skin Sky Drop glitch in Gen 6, and neither occurs.