Disappointing news, but it's difficult to know at the moment what this actually portends.
It's possible that the show was permanently cancelled due to some behind-the-scenes issue that was not made public. Or, it could just be that they had already promised their slots for the next season - who, notably, are all pretty high-profile playwrights like Baker, in their residency program. So they have to keep them happy. It already seems like they're prioritizing their Premiere Residency playwrights, rather than doing their Spotlight or Legacy programming as they often do.
It's odd though, that Hunter, is being allowed to bring back a decade-old play while he is in his Premiere Residency, and that this was prioritized over Annie Baker's play.
If the show has been permanently cancelled, I wonder what that would mean for this play's future. I don't know enough about the industry rules, but wouldn't this play have been the result of a commission from Signature? If so, does that limited Baker's ability to present it elsewhere? I should hope not. She already wrote the dang thing, and she's a pretty major playwright. Hard to imagine it just sitting in a drawer for the rest of her career.
But anyway, it seems more likely to me that, for some reason or another, Signature just has to wait until next season to put it on.