Blizzard says that instead of any sort of loot box mechanics in Diablo 4, they will instead embrace the Season Pass system. This makes a lot of sense seeing as how Blizzard also announced the removal of loot boxes from Overwatch and Overwatch 2 earlier this month.
Blizzard made this announcement as part of its latest development update. They also confirmed that Diablo 4 will be a full-priced title, so while you are free to spend money on cosmetics...