@Mathiason,
@fari, I still didn't readed LN/WN fully but from what was shown that world is some alternative Earth of ~22th century where due to genetic engineering or natural causes like birth of mana a new biological kind of humanity showed up that was able to wield a mana, in some stories those New Humans are alienated as demonkins of sorts and something like that happened here. Afterwards there was a war between the Old and New humans and it might've been spiced up by a global cataclysm and the world degraded after hundreds to thousands of years into medieval feodal society with swords and magic, some rare intact technology like mecha and Lost Items that can't be produced or fixed. Old Humans vere extinct and only New Humans were left while elves and demi-humans that were created by the Old Humans as the soldiers to fight against the New Humans exist till today and oftenly used as sex slaves. It can be our Earth or different one, it's not shown if the game was made with some foreknowledge of this or alternative Earth's future by some God or entity how it shown in similar game titles, when some game is created not just like "here the game and here the same world to it" but to find the heroes or fighters to defeat demons or all the inhabitants of that world, sometimes without "full" isekai by using the trope "they still live on Earth but while playing their temporary avatars are generated in isekai" like in Praise the Orc.
Such tropes of old+new humans, mana+magic, wars and global cataclysms where the Earth stop to look like Earth are used in the stories pretty often, there if few ways how it can happen:
1. In Everyone Else is a Returnee the Earth started evolving and produce a mana, everything living/nonliving started to adapt to it and mutate/transmutate so monster and new materials were born, after each additional evolution monsters become stronger while materials better and environment started to change too. Old Humans might be slow at adapting like humans in some worlds of EER as normal animals and monsters adapt many times faster so becoming extinct due to it isn't strange. Having a mix of monsters, flying islands(after few evolutions) and magic become nothing strange at all, just after a few years after cataclysm the definition of the "norm" changed while almost every human-made object on the Earth was destroyed due to monsters, climate and evolution of environment. At the same time electronics-based technological development can stagnate or fully stop as the factories were destroyed, specialists were lost and new one weren't educated, while materials or/and physical laws might change a bit yet this "bit" will be enough for electronics to stop working or have a need to remake theory about electronics from a scratch.
2. In LVL999 Villager, Yuusha Yamemasu and other stories where worlds suddenly received some "Level System" or were attacked from different dimensions it's somewhat similar to EER - world received a mana right after it happened or gradually in few years(it filled the world from the dimensional rift or bodies of killed monsters, them mana can start generate by itself), monsters and demons were born or started their attack from different dimension, destruction gradually envelopes the globe and after hundreds to thousands years of repetition there can be almost nothing "old" left, knowledge is almost lost and world looks similarly degraded to medieval isekai until some "lost" technology or backstory isn't shown.
3. In Suisei no Gargantia(or trashy movie The Titan, it have the same plot twist as SnG) in the future humanity was on the brink of extinction as there was new ice age - tbe able to live in such a harsh environment and more so in the cosmos to where humanity tried to flee the new kind of humanity was genetically created. It was all fine until the galactic war to full extinction didn't started between both of them thousands of years later, humans had developed technology while "new" humans become some zerg-like monsters that are able to live in cosmos just fine with their own bodies. Coordinates of Earth become unknown, both kinds of humans were able to stay alive on Earth until it warmed up that become a world-ocean without land, human society technology degraded to the level of this title with some artifacts(Lost Items) that can be found undewater while "new humans" lived in oceans like some animals or barbarians and were unable to communicate with the humans yet were antagonistic.
4. In Uwaretemono the world become a harsh place for humans to live and they become extinct except of few MCs with modified body, while their genetically engineered slaves in the form of demi-humans were able to live in the new world and become the new humanity. Except of few places it's just like isekai.
5. In Mahouka Koukou no Rettousei there existed some types of "magical" genes that very few humans previously had, by researching, compiling and genetic engineering a new kind of enhanced humans was created en masse that inherited those genes and were able to use a magic. They were created for wars so normal humans are seeing them as some monsters and the source of danger, there is some possibility of global war between them and normal humans in the furture when there will be more magicians amongst humanity. It you give a century, it can have those mecha and flying islands/ships too, with destroyed Old humanity