Cod, salmon (any size), pufferfish, and tropical fish (any variant) do not spawn naturally in any biome or any part of the world. Non-fish aquatic mobs, such as squid, glow squid, dolphins, drowned, guardians, etc. still spawn normally, and fish who were named or placed from a bucket before the update are still there. 
For reference, this world is in hardcore mode, and was previously 1.21.1, and in that version, fish would spawn normally. I upgraded to 1.21.3 (as in, i skipped 1.21.2) and fish of any kind do not spawn anywhere.
I have a large number of tropical fish in my world, but they are all located in the end dimension, and were manually placed, so they should not be effecting the mob cap for any reason
After some testing, I was unfortunately NOT able to recreate the bug in another world, or even in a backup of the same world, so I am struggling to figure out why this is occuring. The backup is from 3 months ago, so I really don't want to have to go all the way back if i have to.  
Attached are some screenshots I took of the world with hitboxes turned on to make it easier to see any entities. As you can see, there are no fish at all, only the occasional squid or dolphin. If someone asks, I can upload the world.