Good and Evil

Spells that target, have descriptors or domains, or have effects or descriptions related to concepts of good and evil are forbidden by the Three. Schools teaching magical knowledge are not even permitted to teach the concepts related to these spells. Indeed, in general, knowledge of the existence of these spells is lost to the general spellcasting society. As such, most spellcasters don’t even have knowledge that such spells are possible.
Spellcasters can only learn such spells should an in-game opportunity arise that the spellcaster encounters some source that has that knowledge and is willing to teach them. But spellcasters who take on such knowledge also take on the risk of being caught using such knowledge and arrested. Spellcasters arrested with such knowledge will typically just be made to disappear. Better that than such knowledge be allowed to spread via a public trial.
The only exception to this is that clerics of the Three are absolutely forbidden from learning or allowing others in their companionship from learning such knowledge. Should they allow such in their presence, they risk their god withdrawing their power and abilities from them. While this could lead to other opportunities for the cleric, they would be absent all divine magic until such a path plays out. Should a cleric be cast off from one of the Three, they will become outlaw and hunted by those who serve the Three. It is up to the DM to decide if a cleric’s actions have caused one of the Three to cast them off, but it would be appropriate for a cleric to receive a warning in some form from their god before being cut off entirely. This could take the form of visions, guidance from clerical leaders, temporarily weakened powers, and so forth.