- No one ever considers leaving their respective cities unless forced on an adventure.
- There's a suspicious lack of ponies outside the settlements.
With all this taken into account, we can presume the cities are the last remaining vestiges of each race and fled there after the war to protect themselves. This continued on for so long that everyone forgot there was a war and stayed there out of tradition and cultural habit.
However, two know about their history. Spike says upon meeting Rarity that she was far from home and it's interesting that he should know what a unicorn is when no one else did. However, it makes sense considering his sleep lasted a thousand years, so a time before the war.
The other is Kimono. As the resident wisest pony, it would make sense for her to have tales and legends about her history passed down over the generations. Not to mention that her toy mentions knowing of tales when unicorns roamed the forests of Ponyland, which is the name given to the ponies' home in G1.
- Sure. Everything has changed, all with the Core 7 and generation 3.5 semi-reboots. (Oh, should we count New Born Cuties?)
- Isn't it canonically set when the Core 7 ponies were foals? At least the toys were supposed to. That doesn't explain the lack of adults, why everyone looks like a foal, or why there's a mayor so young.