According to "two people familiar with Blizzard's plans", co-founder Allen Adhem was going to announce that Diablo 4 was in the works, but not ready to show during last weekend's BlizzCon. However, it was pulled at the last minute. The rumor comes by way of Kotaku that managed to get off-the-record information from the pair. According to the two, Adham was to make his brief statement at the conclusion of the Diablo Immortal announcement, though no reason why it was removed was revealed.
As most know, the reaction to "only" a Diablo Immortal mobile game has been met with huge backlash with many feeling that a brief mention, similar to what may have been planned, should have been made given the slot Diablo occupied in the keynote address. Usually Blizzard's biggest reveal is placed in the final slot in the overall keynote and this year, it belonged to Diablo. To say fans were disappointed is to understate things greatly.
Read the full report over at Kotaku.