entity falling

I've been trying to figure out the same thing, I've only gotten it to work when a mob is hit personally when making a procedure when a mob falls..

You can do a procedure, change "no additional trigger" in top block to "entity falls", add an if do block with "is [event/target entity] tagged in entity tags as [<your mod id>:<entity name>] as the condition and then make the body of the if do as add slow falling

sorry for poor explanation