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In the Forgotten Realms (before Mystra's ban) the great arcanists of Netheril designed and frequently used epic level spells such as Proctiv's move mountain, a 10th level spell used to raise a mountain and turn it upside down ready to be turned into a floating city.

Have any of the developers mentioned (or even hinted at) the future leading to a lax in Mystra's ban (and therefore an increase in spells possible)? Not necessarily players being able to achieve these spells but rather NPCs like Elminster and arcane casters of similar power.

It's unlikely. Level 20 is defined as a hard cap in D&D 5th edition.

According to a Twitter exchange with Jeremy Crawford:

Q: Could an NPC with class levels get to a character level above 20 as long as they're multiclassed? If 20 isn't the max, what would be their max level?

Crawford: The maximum character level in D&D is 20.

Q: Hopefully subject to change as we get more higher level content over the next few years?

Crawford: If you'd like to keep improving at 20th level, see "Epic Boons" (DMG, 231), which details way to get more powerful at the level cap.

DMG p.231 is quite specific that the hard cap is level 20:

Epic boons can also be used as a form of advancement, a way to provide greater power to characters who have no more levels to gain.

And on page 38 it is even more specific:

Characters who reach 20th level have attained the pinnacle of mortal achievement ... Their ultimate destinies com to pass. A cleric might be taken up into the heavens to serve as a god's right hand ... Characters gain no more levels at this point, but they can still advance in meaningful ways and continue performing epic deeds that resound throughout the multiverse.

This is different to D&D third edition, where NPCs above 20th level were already described in the Living Greyhawk Gazetteer (2000) and Forgotten Realms Campaign Setting (2001).

Support for epic-level play was limited in D&D third edition, as most campaigns never reached that level, which limited the value of content for that tier. Between this, and D&D 5e's more limited release schedule, it's unlikely that we'd see an official Epic Level Handbook, although there are already some third-party and homebrew solutions readily available online.

D&D 5e's epic boons are described as granting such incredible things as a single extra 9th level spell slot, but not 10th level spells or beyond, at least not in the hands of player characters. There's nothing stopping an adventure module from having rituals, artifacts, creatures or even NPCs capable of wielding power beyond 9th level spells, but it's not within the scope of what's available to player characters.

At a panel introducing the first 5e-era Forgotten Realms novels, James Wyatt revealed that the novels would focus on stories of low-level characters, rather than the epic and deific conflicts that appeared during earlier editions of D&D. It seems clear that both in the game and in the world lore in general, the intent for this edition is to focus on ordinary heroes and not beings like Elminster or Mystra.