Kuo said that "the new iPhone waterproof specifications will remain unchanged next year," according to a translated version of the report.
IP codes refer to the degree of protection an iPhone has against dust and liquid. The "6" means the iPhone XS and iPhone XS Max are dustproof, while the "8" represents the highest level of liquid resistance.
IP68 is effectively the highest rating iPhones can achieve under the IP code standard, but there is no limit on conditions like water depth and pressure. 2019 iPhones could have had IP68-rated water resistance up to three meters, for example, but that will not be the case according to Kuo.
This isn't really bad news, as YouTubers have subjected the iPhone XS and iPhone XS Max to a variety of extreme underwater tests, and the devices have generally remained functional. That doesn't mean you should go swimming with your bare iPhone, however, as liquid damage is only covered with AppleCare+.
All in all, this is a relatively minor data point about 2019 iPhones, but one to add to the list as we continue to learn about next year's models.