I played Minecraft a few years ago. The computer at that time was different from now, but I succeeded in getting the world files I played.
Among them, there was a world with an exceptionally large mountain. The height of the mountain is y-255. It includes trees grown in mountains, and if not, it seems to be about y-252.
The game doesn't seem to have collapsed, and the version and exact year are unknown. It was strange that only the world file was left, so I thought it could be illegal copying. Please let me know if this discovery is worthwhile or it's just a bug or collision.
+I re-connected to version 1.8.9, and when I typed the "/seed" command, the seed was "6296462087920619735" and the coordinates were -134 to 624.
This can only be an Amplified world, where such mountains are the norm; otherwise, terrain like this does not generate in default worlds with the sole exception of Savanna Plateau M (now called Shattered Savanna) and even then reaching y=255 is very rare. You can determine the world type by checking the world settings when you select the "recreate" option; I looked at the seed in ChunkBase and the locations in the screenshots corresponds to Mountains (Extreme Hills) (Amplified has the same biome layout as Default. Another possibility is that you have a Customized world with terrain settings adjusted to be more extreme, in which case the preset string, found on the last page of the customization settings, will be needed to replicate it).