Drop doesn't mean "remove from combat". It means dropping them to the ground. So teleport wouldn't work, but a sleep spell might. Perhaps a Spellsword who channels Sleep through their weapon as they attack.
The trouble with the RAW is that they leave it unclear - I'd say anything that allows the weapon to keep going with momentum triggers a cleave. Usually that's the opponent falling down from 0 HP. Using Sleep is functionally similar to being knocked out, so I'd say it would qualify.
But basically, anything other than being killed or at 0 HP is outside of the RAW specifics, and probably falls in the realm of house rulings.
I can see some functional problems if you try and think about it TOO realistically. How does one Cleave with a piercing weapon, for instance?
The trouble with the RAW is that they leave it unclear - I'd say anything that allows the weapon to keep going with momentum triggers a cleave. Usually that's the opponent falling down from 0 HP. Using Sleep is functionally similar to being knocked out, so I'd say it would qualify.
But basically, anything other than being killed or at 0 HP is outside of the RAW specifics, and probably falls in the realm of house rulings.
I can see some functional problems if you try and think about it TOO realistically. How does one Cleave with a piercing weapon, for instance?