The plan was to show a short video presentation on the state of the Diablo 4 project right after the Diablo Immortal unveil, but Blizzard feared that fans would be disappointed even more. Diablo 4 is said to have been in development for 4 years, but the project has changed direction a few times and the current state is not entirely clear either.
https://www.notebookcheck.net/Diablo-4-was-supposed-to-be-announced-after-Diablo-Immortal-at-BlizzCon.357241.0.html