You could extrapolate unofficial higher categories. On average the categories are separated by about 20 mph wind speed. So unofficially we could consider anything above 177 mph to be a category 6. There are 8 on record, including one each in 2017 (Irma, 180 mph) and 2019 (Dorian, 185 mph). But there are six hurricanes that all reached a peak of 175 mph, so we could make the limit 174. That would put Katrina (2005) and Maria (2017) in the unofficial category 6.

But the current proposal is 192 mph for some reason, of which there have been 0 on record. The highest was Allen in 1980 at 190 mph.