Warnings are part of the compiler not the code, and if your project is complicated enough you'll always hit false warnings in some version of the compiler, because some gcc contributor just snuck in a false positive, or a library you're using deprecated something.

Everything being an error makes this worse not better; if -Wdeprecated turns into an error you're just not going to be able to fix everything in someone else's code. Unused code being an error is easy to fix, but it's so disrespectful. I shouldn't get compile errors as I'm editing when I haven't done anything wrong.