A homing projectile that will only target the entities that are poisoned
- class: CustomProjectile
target_potioneffect: POISON

And in parameters:
track_target: true

You might wanna look at how betonquest does this, its an open-source plugin:
https://github.com/Co0sh/BetonQuest/wiki/Conditions-List
look for potion effect condition there.

I think this could be done as a base spell targeting filter, so it could be used for homing in projectiles or any other targeting.

Well nearly a year later... I've added target_potion_effects and ignore_potion_effects parameters.

Both take a list of potion effect types. The target parameter will target entities with any of the given potion effects, the ignore parameter will skip entities with any of the effects.