Title says it all. There is no longer any ability to use the `fish` option in the `/loot` command. Whilst it was never a very useful feature, as it stands, it's just a completely non-functional feature within the game. I took some screenshots at various levels of user-support. The command used for each was various spins on:

/loot give @s fish minecraft:gameplay/fishing/treasure ~ ~ ~ mainhand

Various levels of user support for each screenshot:

1. Running the command as is.
2. Running the command whilst holding a fishing rod.
3. Changing the coordinates of the command to be the top (also tested a block above the top) of a 5x4x5 pool of water.

Each of these resulted in the red "An unexpected error occurred trying to execute that command" message. My theory is that the changes to how fishing works as of 20w12a accidentally disabled this functionality and given how very rarely people use it, it wasn't noticed until now.

As per the first commenter's suggestion, I tried this in a river and then in an ocean with absolutely no change.