If you are using #pragma option(pedantic), then you are telling Slick-C to flag any deprecation as an error. Slick-C doesn't have warnings.
The docs should be updated. I'll make a note of that.
And, yes, _isUnix() is the correct replacement for #if __UNIX__. _isWindows() is the replacement for #if __WINDOWS__, and _isMac() is the replacement for #if __MACOSX__
We do our best to keep macros backward compatible, but #pragma option(pedantic) is a special case (and documented as such). If you use it, you are subject to future language changes, symbol deprecation, and higher levels of error checking to cause your existing macro to no longer compile. We use pedantic in all the code we ship, but recommend #pragma option(strict) for users.