In vi, df and dF are inclusive (in that they delete the character the motion is targetting) and dt and dT are exclusive. Right now, in fish, all of these commands delete one character less than they do in vi.
I looked into it a few times myself, but the last time that I sat down to do it I was put off by the assumptions neovim makes about the purposes behind the integration. The RPC api is (was?) very restrictive in terms of how an integration must be approached and the kind of state that would be kept client-side, so I ended up tabling my efforts.
This off-by-one inconsistency also applies to selecting text in visual mode: in vim, the selection includes the character under the cursor. In fish, the selection includes up to but not including the character under the cursor.