Between 1.14 and 1.16, Ghasts were changed internally from being a "Mob" to being a "Monster". By default, monsters only spawn when the light level is low. (This is hardcoded in the game and cannot be changed by json-based spawning rules).
Expected Behavior: Ghasts spawn at all light levels.
Actual Behavior: Ghasts only spawn if the light level is below 8.
Consequence: Portal-based ghast farms no longer work because portals emit light.
Demo world: Press a button to switch from a netherwart block floor to a shroomlight floor.
How to fix: Add a Ghast::isDarkEnoughToSpawn() function that always returns true. Example code below. This wasn't needed before when Ghasts were Mobs, but now is needed because they are Monsters.