Blizzard Game developers have announced that their latest Diablo 4 game will not have an offline mode.
But instead, you can play solo and dungeon mode in private. Campaign quest can also be played privately like before.
However, People who have played Diablo 3 never complained about the unavailability of an offline mode so why care about adding one? Also, most of Blizzard's other games like World of Warcraft, Overwatch, and StarCraft don't have an offline mode.
But console versions of Diablo 3 do have an offline mode. Diablo 4 is still in the making for PS4 and Xbox One platforms along with PC but an offline mode is not on the cards for any of the platforms as mentioned.