Russian Lostbelt : Basically a meteor crashed in the 1600s and caused an ice age. Humans were forced to fuse with demonic beasts in order to survive the intense cold, becoming a new species called the Yaga. However, this had the trade-off of requiring a ridiculous amount of food to maintain their body temperature. This mean that they had to literally spend all their time hunting demonic beasts for food. I am not kidding, they didn't have time to build any additional housing at all. Needless to say, when humanity can't even build houses, it is stagnant.
Scandinavian Lostbelt : The Fire Giant Surtr sought to break the prophecy of Ragnarok and destroy the whole world. He ate Fenrir, and basically kicked everyone's asses until Odin trapped him in a fake sun and appointed Scathach-Skadi as ruler before kicking the bucket. Under her rule, all that was left of humanity was a carefully controlled population trapped inside two dozens of villages, with adults and excess villagers being sent to be eaten by the giants to keep them docile. Their population basically never changed, and neither did their technology.
Chinese Lostbelt : China conquered everything after the emperor actually got immortal instead of drinking lead. Their tech level soared, and like 99% of the planet is dedicated to farming to produce enough biofuel to keep the emperor super-computer active. They even had two orbital rings around the planet to defend against any alien invasions... and that's all they did. They never even tried to leave Earth's orbit, and everyone with enough braincells to actually figure things up are stuck in the capital drawing portraits of their emperor.
Indian Lostbelt : Arjuna Alter ate most of the Indian Pantheon (Only ones that weren't eaten were Kama, Ganesha, & Parvati) and decided to speed up a bit the whole cycle of Death & Rebirth that the Indian Mythos rely on, in order to remove all evil and imperfection. He sped it up too much (Granted Limbo convinced him to go even faster to one week cycles) and humanity couldn't keep up anymore. Not to mention that his definition of Evil boiled down to any imperfection. Were you wounded during the monster murder-fest that happens on the last day of the cycle? Well too bad that wound is an imperfection and Arjuna will erase you when he destroys the world and remake it. It is pretty clear that the whole place was going toward self-destruction anyway.
Atlantis/Olympian Lostbelt : A very long time ago, Sefar the White Titan came down, and basically killed all of the Gods, downgrading them to Divine Spirits reliant on Human Faith and starting the decline of the Age of Gods. Excalibur, the holy sword of King Arthur, was specifically made by Planet Earth to kill Sefar before it killed everything on Earth. For those wondering why the hell Sefar was doing that, it's basically the Nasuverse equivalent of the Reapers in Mass Effect. Anyway, in PHH the Greek Gods, who were actually AI spaceships from another universe, stayed in their own territories and were destroyed one by one by Sefar, with the one giving the White Titan the hardest time being Ares. The terminals they made to interact with humans were still alive, and became Divine Spirits.
In the Lostbelt, Zeus used his literal admin powers to force all the 12 gods to fuse into a giant mecha and fight Sefar. They didn't win, all they could do was last long enough for Excalibur to be made and for Sefar to get blasted, but their bodies stayed intact. They never were degraded into Divine Spirits like the rest of the Gods, and they were unaffected by Human Faith, nor did they rely on it.
It's getting very long, so to summarize after that : Zeus went on to give Humanity lots of awesome tech and turned Olympus into what is basically an ant-farm. Humans physically couldn't progress any further, everyone was immortal, e.t.c. Literally nothing changed for thousands of years. As one character put it : "Today is the same as yesterday, and tomorrow will be the same as today.", which is why it got pruned.
British Lostbelt : Remember Sefar? The White Titan that tried to destroy the Earth and got stopped by Excalibur? Well in this timeline the fairies in charge of making Excalibur decided to skip work, resulting in Sefar turning the entire planet into an endless ocean without any life nor landmass. Eventually the fairies came out into the endless ocean and got bored, and eventually met the god Cernunnos, who hid in the Reverse Side of the World with his human priestess to wait out Sefar. He came to punish the fairies for, y'know, not doing their jobs, but he took pity on them and let them live on top of him. Because fairies are bastards, they eventually weren't satisfied and killed Cernunnos by poisoning him, and when his Priestess tried to opposed the fairies, they cut her up into pieces and used their magic to keep every piece alive, doing the magical equivalent to cloning to get a lot more than one human. But, in the Nasuverse, just because a God is dead doesn't mean that they are completely dead, and eventually a Calamity arrived. Basically curses from Cernunnos in an attempt to kill the fairies for killing him and giving his human friend a fate worse than death. That's when the fairies decided to leave. Because you see, when a fairy dies, its body become terrain, and eventually new fairies start popping up. So the British Fairies just started murdering each other, pilling enough corpses to make a new landmass which, because stuff across timelines is oddly similar to each other, became Britain. But Britain wasn't happy to be created. After all, it was a literal pile of corpses in the shape of the real island, with a dead god at its center that was also cursing the fairies. So more calamities happen and eventually all the fairies dies and the Lostbelt is a desolate wasteland. Something happen that changes that, but I won't spoil it.