According to a report from VG247, an interview with Blizzard’s Wyatt Cheng (who is the lead game designer for Diablo Immortal) revealed that there is no offline mode for the game, and that players will be required to have an active internet connection on their mobile devices if they want to play the game. Players can play solo if they choose, but ultimately an internet connection is still required.
Most games these days do require players to have an active connection, although we’re sure that some were probably hoping that there could be a single-player mode for the game that would not require players to be online (which can be draining on their battery/data). Not much is known about the game at the moment, such as pricing and a release date.
However many Diablo fans seem to have reacted negatively to the game’s announcement, but whether or not their sentiments will change when the game is released remains to be seen.