While searching through the default Minecraft textures, I found an image I didn't recognize. If you have default textures saved on your computer, go to assets\minecraft\textures\gui\presets\ and there is one image with the name "isles.png". This is the image:

I had never seen this image in-game before. I tried a google search, best thing I found was someone asking the same question on Reddit with no replies. From checking the game files, it exists in every version since 1.8. However, before the 1.15 snapshot 19w45a, there was another six: chaos.png, delight.png, drought.png, luck.png, madness.png, and water.png. While looking at the changelog for that snapshot, I learned they were from customized world type preset icons, which explains why they were removed. Yet they left isles.png for seemingly no reason. Does anyone have any idea why they would keep isles.png but none of the rest?