In Forgotten Realms lore, can gods use spells above 9th level?

Question

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.

Answer 1

Yep, they definitely can

Looking at the 3rd Edition Forgotten Realms Sourcebook Faiths and Pantheons, we get this as a listed Salient Ability that a god may have (i.e. a "things a god may be able to do because they are a god")

Divine Spellcasting:

The deity can cast spells whose effective level is above 9th. The number of spells per day the deity can cast of each spell level above 9th depends on the deity’s key ability score for spell-casting classes in which the deity has achieved 20th level

Following is a table listing the required ability scores for the deity to have spell slots up to 25th Level. With a note that says

Even though the table only includes ability scores up to 61 and spell slots up to 25th level, the progression continues infinitely in both directions. For ability scores beyond 61, or for spell slots above 25th level, expand the table to follow the same patterns as shown.

The can be found transcribed in the 3.5e SRD

I would also add, however, that deities don't have to use spells to achieve supernatural feats if they don't want to. Another Salient Ability available to them is...

Alter Reality

This ability is similar to the wish spell. The deity merely thinks of something and then makes it so.

This is not the deity casting Wish...it does not come with the restrictions or perils that a mortal wielding the Wish spell are saddled with. This is a god altering reality because they are a god who is able to do that. (This is still a drain on their power and requires them to rest after using it...which is why gods aren't constantly doing this. But, y'know...when a Cleric prays for a Miracle and something not replicable by a spell happens? This is probably what the god did.)

Spellcasting is a god electing to use the interface created by Mystryl/Mystra that let mortals safely interact with magic.

Suddenly, rather than magic being restricted to just the deities, any creature with the talent or training could draw upon the power of magic through the Weave.

Magic of Faerun p.4

Deities were perfectly capable of wielding magic before The Weave existed--it's a useful interface that some of them choose to interact with, but they can manifest magical effects because they are gods--they don't need The Weave to do it.

Answer 2

According to the AD&D book, Secrets of the Magister, it is still possible for mortals to research and cast 10th level spells

In the Forbidden Magic section of the Magisterial Magic chapter of the AD&D book Secrets of the Magister (published in 2000):

After the Folly of Karsus [...] , the newly ascended Mystra stripped mortals of the ability to handle specific 10th level spells.

[...]

History incorrectly records this as changing humans to make the use of such magic beyond their grasp, or changing the nature of magic to make spells of greater than 9th level unintelligible to humans, because that is what the goddess wanted mortals to believe[...].

It is possible, in the realms today, to research True Dweomers, and even new 9th level spells to achieve specific, severely limited ends that resemble parts of what a 10th level spell could achieve. [...]

[...]

The information related in this section is largely secret, but the points about the possibility of researching powerful magic, and the restrictions on the casting of found 10th level spells, will be slowly and partially shared (through visions shared by Mystra) to any player character who prays to Azuth, Mystra, Oghma, or Savras for answers on this topic - or by high ranking clergy of Azuth, Mystra or Savras who are directly asked about such things by any mage (such is Mystra's commandment).

There are pretty severe restrictions on mortals attempting to cast found 10th level spells which I'm not going to reproduce here, but those restrictions apply only to those specific banned forms of the spells that existed at the time of the ban.

Since it is possible for mages "today" to research (and cast) True Dweomers, it follows that it is possible for gods to do the same.

It is unclear of whether or not Mystra would be able to place restrictions on the magics other gods can effect.

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Note: it is unclear whose voice this book is written in (many of the books written have an in universe voice as the writer), it was written by Ed Greenwood (creator of the Forgotten Realms). As such, unless directly contradicted by a later source, I believe this to be the authoritative lore on 10th level spells as they pertain to the Forgotten Realms.