Finding mineshafts out in the open is rare, especially in plains biomes. The only good explanation to why there could be a mineshaft displayed out in the open in a plains biome is likely because it’s situated very close to where a mesa biome starts. And as you might know, mineshafts and gold mines are two common aspects of mesa biomes.
However, when we checked out the seed a little bit more thoroughly the mineshaft continued in the opposite direction of the mesa biome, instead stretching further out in the plains biome.
In the image below you can see the open mineshaft in the plains biome and the river dividing it from the mesa biome.
If you decide to go on and explore the mesa biome you will find several epic views such as this one with multiple mountain islands in a large lake.