Based on the list of exhibitors and map released earlier this month, it appears that Blizzard Entertainment will not have a booth at the San Diego Comic-Con this year.
Blizzard has been a big presence in previous years, with a massive corner booth often offering exclusive merchandise such as the coveted Nether Faerie Dragon and Whimsyshire Treasure Goblin plushies, several of which never make their way to resell elsewhere, making them very coveted collectors items. Even the cancelled SDCC 2020 released a new line of summer collectibles on the Gear Store in lieu of the in-person event.
Blizzard has released several Comic-Con exclusives over the years, several of which are now fairly rare.
No specific reason or announcement regarding their absence has been made, though Blizzard is still set to appear at Gamescom in August, so they haven't pulled out of everything.