Simple dark rooms, ie darkened areas that promote mob spawning, are legal.
The following are prohibited:
- Mob grinder mechanisms, including piston traps, lava traps, and structures that cause damage to mobs (ie fall damage)
- Structures that siphon or direct the movement of mobs
- Structures that protect players from damage while fighting mobs
- Afk spawn killers (mechanisms that kill mobs while players are afk).
The following are acceptable:
- Structures that protect players while healing (not fighting)
- Using projectiles from outside the mob aggression range.
@Xargun The rules well always be your first source. But yes darkrooms are legal but do have limitations.