You've found them all.

The Forgotten Realms was first published for AD&D 1st edition. When 2nd edition came out, the change in spells and available classes was explained through the Godswar aka The Time of Troubles.

The introduction of D&D 3e was exceptional in that there was no overarching, global in-setting event introduced to explain the rules changes. Oddly, the adventure Die Vecna Die! was intended to explain the rules changes (for all D&D settings), but its events were never made FR canon. WotC just proceeded with the 3e rules and setting changes without adding any corresponding historical events. The 3e change is thus “silent” in FR's historical record.

Karsus's Folly was made global setting canon with 3e as a historical detail that further developed the history of Netheril and explained some changes local to the Anauroch and some new game elements, but it wasn't the same kind of global upheaval—it was more of a retcon slipped in as undisruptively as possible.¹

The Spellplague (and about a century) explained the shift from D&D 3.x to 4e, and the Second Sundering did the same for the transition to 5e.

1. ^{Although Karsus's Folly was first described in a 2e adventure, the way 2e setting publishing worked meant that only the frozen-moment-in-time documented by the basic setting boxed set was canon. DMs were given explicit authority over what was canon beyond that. Thus changes made by adventures were only canon in a campaign if a DM used those in their campaign — because of course, the PCs might change the outcome! D&D 3e cemented Karsus's Folly as the official way it happened in Realms history for that revision of the base setting, so though it took place earlier, it wasn't canonised until 3e.}

They're there for you, the DM, to create something.

The usual complaint about the Forgotten Realms is that too much is already written, making it hard or impossible to learn the setting for DMs, let alone the poor players, or for DMs to fit in anything of their own.

Having "blank lands" like this scattered around the setting leaves room for DMs' own creativity. If you have an idea, but can't fit it into one of the more popular playing regions because the lore and canon conflict with what you want to create, these blank lands are there to give your idea a home.

As a point of history, Sembia (south of the Dalelands) was a large blank area right in the middle of one of the most heavily-developed regions in the Realms, in the original AD&D 1e Forgotten Realms Campaign Set. It was set aside for DMs so that they could place their own kingdom (or expand a home game into a Forgotten Realms game by locating their existing campaign kingdom there) without having to settle for a region far away from the majority of Forgotten Realms setting material.

TSR promised to never develop Sembia so that DMs' creations would not be interefered with. That lasted all of six years before Sembia was written up in the AD&D 2e Forgotten Realms Campaign Setting, and a bitter hue and cry was the result.

So thank WotC for leaving you these blank areas. They gave you a name and an evocative description to set fire to your imagination — now follow your dreams and create something awesome that your players will love.