This is from a world I created in 18w45a. After playing in 18w46a for a while I started to notice that I was no longer seeing any fish in a nearby lake and after exploring it for a while I didn't see any spawning further out. A fish I caught and put in a little pond didn't seem to be affected, so this seems more like a spawning issue.

Update: After spending some time away from my main base exploring, I noticed fish started spawning again. After returning home, I found a single fish swimming in the nearby lake, but it despawned shortly after and I haven't seen anything since.

I think what might be going on is something to do with the new pillagers maxing out the number of creatures that can spawn in a given area, or maybe they're taking priority. There certainly seems to be a lot of pillagers spawning. Like two or three groups of them in a given area at a time.

Update 2: Whatever was going on doesn't seem to be happening now, at least not to the same degree. I don't know, maybe saving and reloading Minecraft a couple of times forced it to work through whatever kink was keeping it from spawning fish.