I opened my world from 1.13 in the latest snapshot and mobs rarely spawn in the overworld. The nether and the end seem to be normal. Guardians aren't spawning in the ocean monument. I left my player in an unlit area for three or four in game day and night cycles and nothing spawned. There is nothing in the spawn chunks that would take up the mob cap. Mobs will spawn when there is a dungeon with the mob spawner, but that is all.