After Karsus’s folly, Mystra imposed a ban on use of magic above 9th level (I believe there are specific conditions for 10th level spells, but that’s not the main issue here). However, I believe the specific mentions of Mystra’s Ban have unclear wording:
In the aftermath of Netheril’s fall, however, Mystra banned certain high-level spells that she deemed too powerful for mortals to wield responsibly. Thus, current-day spellcasters no longer have access to true spells of 10th level and higher. Instead, access to epic magic comes via two feats—Improved Spellcasting Capacity and Epic Spellcasting—that function in very different ways.
(p43, Lost Empires of Faerun)
When Mystryl reincarnated herself—this time as Mystra—she used the form of a beautiful peasant girl learning the basics of cantra magic but with the capacities for archwizardry. Her first priority was to recreate the weave of magic. This time, she made magic follow a few more rules, and no spell above 10th level would function.
(p12, Netheril: Empire of Magic)
The second text seems to imply that the weave doesn’t allow for spells above 10th level. Which means that the gods, if they use the weave, couldn’t either. The first one, however, says that this ban is due to mortals not being responsible enough to use those spells. So perhaps the gods, who haven’t shown such irresponsibility, can use them?
I fear that other source material isn’t particularly optimistic. The Deities & Demigods book for 3e doesn’t mention the use of spells above 9th level.