Did someone forget to add fish textures for 1.8.9?

I mean why wasn't it upscaled to 32x

Because nobody made the texture yet.

This was done on purpose. Seeing as Jappa himself made this texture, the general consensus is that the colour change of the fish texture in 1.13 is more specific to the Update Aquatic rarher than being an integral part of the Texture Update, i.e. that it wouldn't have happened if the Update Aquatic didn't. As a result, the old, more generic fish texture will keep its green colour in versions 1.12 and below.