Mobs that are immune to oozing and infested through datapacks will only be immune to infested.

To reproduce:

1. Enable the attached datapack, adding chickens to the #immune_to_oozing and #immune_to_infested entity type tags.
2. Spawn a chicken
3. Splash the chicken with Oozing
4. Kill the chicken
5. Note that slimes spawn even though chickens are immune to Oozing through the datapack

Code analysis:

LivingEntity.class

public boolean canBeAffected(MobEffectInstance mobEffectInstance) {
    if (getType().is(EntityTypeTags.IMMUNE_TO_INFESTED)) {
        return !mobEffectInstance.is(MobEffects.INFESTED);
    }
    if (getType().is(EntityTypeTags.IMMUNE_TO_OOZING)) {
        return !mobEffectInstance.is(MobEffects.OOZING);
    }
    if (getType().is(EntityTypeTags.IGNORES_POISON_AND_REGEN)) {
        return !mobEffectInstance.is(MobEffects.REGENERATION) && !mobEffectInstance.is(MobEffects.POISON);
    }
    return true;
} 

If the entity is in the #immune_to_infested tag, the code only checks if the applied status effect is infested, and does not check whether the entity is also immune to other effects.