This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
Between is a joke biome introduced in Java Edition 20w14∞. It generates in the "fleet" dimension.
The biome consists of a somewhat mountainous terrain made out of stone and a large fleet of end ships in a grid pattern generating both in the sky and the underground. No mobs spawn here.