I know MobEffects initialization. But he asked about
"I've seen around the internet that they go Potion.getPotionByID(), or was it PotionEffect.getPotionByID()? Either way, it doesn't seem to be supported here."
So I answered him same way. Sorry bro but knowing "MobEffects" init doesn't make you good or bad modder. Anyway here is the code with "MobEffects" for "GOOD" mooders...
player.addPotionEffect(new PotionEffect(MobEffects.POISON, 100, 0));