OK, just to add some more context to this issue and show it's not me just complaining about a mod not dropping.
Void Hole is meant to drop from Exo Outrider. It's definitely spawning as I have four copies of Fiery Phoenix which is it's other 0.15% mod drop.
In terms of other mods that have the same low drop rate of 0.15% I have:
Forward artillery = 2
Section Density = 1
Void Cloak = 1
Particle Ram = 7
Seeker Volley = 6
Fiery Phoenix = 4
All of these drop in Veil yet what appears to be going on with Void Hole is inconsistent. Given the number of games I should have seen at least 1 Void Hole drop unless something is broken.