This report specifies over 6 blocks, though it seems to have changed to over 7 blocks normally and over 4 for dripstone. Falling from 8 blocks without a horse causes 2.5 damage, falling from 8 blocks with a horse causes 2.5 damage. Falling on to dripstone from 5 blocks or more still causes the same damage with or without a horse.
Can confirm this is still an issue, found a teal-yellow dasher in the wild so the colour + type combo is available, but it is not specially named nor does it have the increased spawning chance
This report specifies over 6 blocks, though it seems to have changed to over 7 blocks normally and over 4 for dripstone. Falling from 8 blocks without a horse causes 2.5 damage, falling from 8 blocks with a horse causes 2.5 damage. Falling on to dripstone from 5 blocks or more still causes the same damage with or without a horse.