Every certification card says the same thing and almost nobody is told why. The short version: your body is still off-gassing nitrogen for hours after you surface, and a pressurised cabin is not sea level.
What is actually happening
Under pressure, nitrogen dissolves into your tissues. On the way up it comes back out - most of it quickly, some of it over the following hours. A commercial cabin is pressurised to somewhere around 2,400 m, which drops the ambient pressure again and speeds up the release of whatever is left.
The numbers
A single no-decompression dive: twelve hours minimum. Multiple dives in a day, or several days of diving: eighteen hours, and this is the case for every trip we run. Any dive requiring a decompression stop: at least twenty-four.
Why we plan around it rather than warn about it
Because the alternative is a diver with a 6 a.m. flight asking for one last dive at the jetty, and the honest answer being no. Our itineraries put shallow dives on the last dive day and finish before midday, so the question never comes up.
The same applies to the road to the airport if it crosses a pass, and it applies whether you feel fine or not. Decompression sickness does not announce itself in advance.