Weekly bonus caches from world bosses no longer have a level requirement for opening.
There are currently 3 World Bosses in Diablo IV: Ashava the Pestilent, Avarice, the Gold Cursed, and Wandering Death, Death Given Life. They all have multiple spawn locations on the map, and will only appear on the map 30 minutes before they spawn. Additionally, the map marker for World Bosses is tied to Whispers of the Dead, an endgame activity only available once you’ve beaten the main story (or are using a character that has skipped the campaign). This marker is a diamond inside a red spiky circle (the outer shape is the same one used for other Whispers of the Dead tasks that reward 5 Grim Favors). If you’re in the same region as the world boss, a notification will also appear at the top of the screen once the 30 minute timer starts.
Technically, it is possible to still fight World Bosses even if you have not completed the campaign, assuming you learn its location via other players. However, you wont earn any Whispers of the Dead progress, and you’re level will likely be too low to do much damage.
World Bosses are affected by the World Tier you’re currently in. World Tier World Bosses have a minimum level of 50, level 55 at Tier 3, and 60 at Tier 4. If you’re higher level than the minimum requirement, then the boss will scale to your current level. Additionally, World Bosses also change their stats based on how many players are currently in the boss arena. Meaning that if you have a few underleveled or undergeared players, then the fight will become harder as a result, compared to having less, better prepared players.