Havard posted that topic, because one of a set of videos on high level spells (from the Netherese Empire) mentioned Spelljammer, and MrRhexx spoke enough about Spelljammer for the spell description to make sense.
Here are MrRexx's videos on Arcane Age spells:
- What They Don't Tell You About 10th level Spells - D&D
- What They Don't Tell You About 11th Level Spells - D&D
- What They Don't Tell You About 12th Level Spells - D&D
- How To Cast 10th Level Spells - D&D
List of 10th Level Spells:
- Lefeber's Weave Mythal: (Invocation/Evocation) Field: Inventive/Mentalism - Encyclopedia Arcana page 10
- Mavin's Create Volcano: (Conjuration/Summoning) Field: Inventive - Encyclopedia Arcana page 10
- Mavin's Earthfast: (Alteration) Field: Variation/Inventive - Encyclopedia Arcana page 11
- Moryggan's Mythaleash: (Conjuration/Summoning, Evocation) - The Fall of Myth Drannor page 56
- Proctiv's Move Mountain: (Alteration) Field: Variation - Encyclopedia Arcana page 11
- The Srinshee's Spellshift: (Abjuration, Alteration, Divination) - The Fall of Myth Drannor page 61
- Tolodine's Killing WInd: (Alteration) Field: Variation - Encyclopedia Arcana page 11
- Valdick's Spheresail: (Conjuration/Summoning) Field: Inventive - Encyclopedia Arcana page 12
- Mavin's Worldweave: (Alteration) Field: Variation/Inventive - Encyclopedia Arcana page 11
- Proctiv's Breach Crystal Sphere / Proctiv's Seal Crystal Sphere: (Alteration) Field: Variation reversible - Encyclopedia Arcana page 11
- Karsus's Avatar: (Invocation/Evocation, Enchantment/Charm, Alteration) Field: Mentalism - Encyclopedia Arcana page 10
And there are only actually a handful of spellcasters who ever learned how to create a 10th-12th level spell:
- Karsus (Karsus’s Avatar 3519 NY)
- Lefeber (Lefeber's Weave Mythal 1741 NY)
- Mavin (Mavin's Worldweave 3205 NY, Mavin's Create Volcano 3258 NY, Mavin's Earthfast 3349 NY)
- Moryggan (Moryggan's Mythaleash ?)
- Proctiv (Proctiv's Move Mountain 1746 NY, Proctiv's Breach Crystal Sphere / Proctiv's Seal Crystal Sphere 1808 NY)
- The Srinshee (The Srinshee's Spellshift ?)
- Tolodine (Tolodine's Killing WInd 1500 NY)
- Valdick (Valdick's Spheresail 2028 NY)
The other spells were around for 170-2000 years, so the number of spellcasters who were able to access and learn the spells would have varied a lot.
The official Netherese attempts to expand into Realmspace and beyond ran from 2795-2895 NY (which is kind of weird, because it's 900 years after Proctiv created a spell that would only work on a crystal sphere and 770 years after Valdick created a spell that would only work in Wildspace and the Phlogiston).
Yeoman's Loft continued as a spelljamming port until the Fall, but only five ventures were using the port.
So it's not impossible for any of the other spells to make it off-world and therefore also to other parts of Toril. But I think the probability of other copies of spells existing is going to be higher for earlier spells and specifically spells created before 2895 NY.
Having said that, if Netheri: Emprire of Magic says that Mavin's Create Volcano has never been cast in Netheril, I wouldn't be surprised if Mavin chartered a spelljamming ship specifically to transport him to a place where he could cast the spell to see it working.
LAST EDIT: 10th level spells from The Fall of Myth Drannor added.