I am playing Minecraft 1.12 but today the in-game textures look much lower quality than usual. I don't have any of the super secret settings turned on, and I'm unsure how to fix this and get the normal textures back.
This was an update released with April Fools day, and is only available to 1.12 clients. You can read about it here: https://minecraft.net/en-us/article/try-new-minecraft-java-textures
Don't worry, this got me too. I forgot it was April 1st, everything should be fine by tomorrow.