But it's referencing a component that no longer exists on the form. This always used to come up with the warning and ask if you want to remove the associated declarations.
No, its not.
The only "reference" is in the name: BitBtn1Click
But that is just a name. (and names have no meaning to the compiler - except different things must have different names)
The "OnClick" event could have any name you choose. You could have called it: "GetMeACoffeeProcedure" and it still would have handled the click.
The procedure is linked to the BitBtn by the form designer / object inspector. That is saved in the lfm file. And when you deleted the bitbtn, that link was deleted too.
(Look at the events of your new Button, to see how this is linked to its event)
Now, it is just a method, that is not called at all. But the compiler does not know that (the compiler does not check what is in the lfm, that happens later). You could still have plans to do something with that code.