As a part of today's 1.1.0 Patch, Blizzard has described the changes to how experience will be rewarded when Season of the Malignant launches on July 20th. These changes include reducing the bonus experience from killing higher-level monsters, changing the bonus experience gain ranges, and limiting the experience that can be gained by party members, based on monster level. Diablo 4 Patch 1.1.0 Notes Patch 1.1.0 Official Blog Post
Bonus Experience AdjustmentDuring Preseason, players could gain an experience bonus from killing monsters that were up to 3 levels higher than their character level. These bonuses are currently:
Pre-Season bonuses:1 Level higher: +15%. 2 Levels higher: +20%. 3+ Levels higher: +25%.When Season of the Malignant starts on July 20th, players will gain less bonus experience for killing higher-level monsters. However, the monster level range that players can receive this experience bonus from will be increased:
Season of the Malignant:+1 level: +1.5%. +2 levels: +3%. +10 levels: +15%.So, while the experience bonus is decreasing in general, players will be rewarded for conquering enemies that are much higher level than their character.
Powerleveling NerfCurrently, players are able to group up with a higher-level character in order to be carried through Capstones and high-tier Nightmare Dungeons. The way that this is accomplished is by the high-level player going into the Dungeon and killing all of the enemies, while the low-level player waits at the entrance and gains all of the experience. While this leveling method will still be possible, experience gains will now be capped based on the specific character's level:
Current World Tier Level offsetsWorld Tier II: Up to +3. World Tier III: Read more on wowhead.com