Apparently that's new: in the US at least, Tdap was only approved in 2005, and replaced Td (the previous standard bundled tetanus/diptheria every-10-years booster) in most recommendations/clinics around 2006-2007. So adults who haven't had a tetanus booster in the past 3-4 years didn't get the one that includes whooping cough, and probably don't even know about the change (I got my last booster in 2003, and didn't know that it was different now).