Ah, I got it mixed with Pathfinder 2e. Then again, are there any Player-side abilities that cause damage/drain? It might just be so that it's a one-way street with only monsters/NPCs causing it on PCs.
I don't think so... and I'm pretty sure it's vanishingly small on the monster side too.
Monsters don't have to qualify for feats.
So even with a player ability causing ability score drain, it would only make any difference if the monsters had an odd number score at the end.
Just assume that the monsters have an even numbered ability score to begin with and drop the modifier by one for every two points of drain.