Warning: contains major spoilers for Infinite Frontier #2!
The last time the Justice League saw Wonder Woman, she was sacrificing herself to save the universe. Now, in Infinite Frontier #2, it has been revealed that the superhero team hasn't been forthcoming with this information. It isn't public knowledge that Wonder Woman is dead.
Dark Nights: Death Metal saw Diana's time as a hero end in the mortal realm. After her final sacrifice, she was approached by the Quintessence - a group of six beings in the Multiverse who are god-like observers. They offered her a position among them, a "transcendence" past mortality. She turned this down and has since been in Valhalla trying to get back to her former life, though Wonder Woman #780 this October will see her return home.
Created by the team of Joshua Williamson, Xermanico, Jesús Merino, Norm Rapmund, Paul Pelletier, Romulo Fajardo, Jr., and Tom Napolitano, Infinite Frontier #2 suggests that the Justice League knew she would be coming back - or at least that they hoped she would be returning home. While making inquiries at the Hall of Justice, one of the many questions Agent Chase wants answers to is: where is Wonder Woman? Agent Chase is working with Director Bones. Like many of her other questions, this one is also ignored, but it raises an important point: the Justice League hasn't revealed Wonder Woman's death.
Superheroes die and are resurrected all the time, but Diana is part of the DC Trinity. Along with Batman and Superman, she has one of the highest profiles in the public eye. Her absence would certainly be noticed. However, there are plenty of believable reasons that her absence could be covered without revealing her previous fate. As a hero whose world has gotten bigger, it wouldn't be hard to suggest that she is saving the world somewhere else, or in another multiverse - now that it has been revealed to the general public. DC heroes face threats all across the universe and have for a long time. Nonetheless, the reformed supervillain in charge of the DEO (Director Bones/Department of Extranormal Operations) has noted her absence. Yet that doesn't answer why the Justice League has kept her death a secret.
One reason could be for public relations. As mentioned before, Wonder Woman has a high profile. She is a well-known hero and her death could cause more strain on the Justice League's reputation with the public. However, more likely than not, they have assumed that she will be returning eventually. The Totality, who has been working with numerous DC heroes, is aware of multiversal changes that include heroes returning from the dead. It wouldn't be difficult to believe that if other heroes are suddenly resurrected, Diana could eventually turn up among them. Additionally, she is one of the League's strongest members and not much is able to keep her down. Death isn't always what it appears and if DC's heroes are familiar with their history and past threats they've faced, they could expect that she hasn't actually died.
Ultimately, this shows the level of faith the Justice League has in Wonder Woman, as well as the uncertainty of the future. Though they aren't necessarily sure of what multiversal changes are coming, it wouldn't seem worthwhile to make her sacrifice public - just to have her show up the next day as though nothing happened. It seems easier to cover up a superhero's lengthy absence than backtrack and explain how a dead hero has returned without causing confusion and even further distrust in the Justice League. Wonder Woman WILL be back, so keeping her initial death a secret is a gamble that will pay off.