During long drives with the engine at steady state (interstate highway at constant 75 mph for more than one hour) I've noticed the water temp will sometimes slowly rise from 90C to 95C over say 30 minutes; and then it may trend down. This in Sport mode. No alarms, warnings, messages or changes in operation or coolant level. I attribute it to the ECU doing its thermal management thing. Many long road trips; has not been a problem.

 

I have not seen this. I'm steady at 194F all the time. FWIW, I'd suspect leaves building up in the radiator area before coolant.

 

Check for water pump leaks or solenoid/COV that controls the coolant temp.

 

I did some testing few weeks ago.
Our dashboard/system is preprogrammed to show 90 C (or 194F) and not show higher data. It is also on other cars from VW group (my friend with VW Golf R had the stop at 90C as well, but went to ECU specialist for modification to show correct data).

Im surprised you have seen higher temps on dash.
However, when you plug in diagnostics and do some data logging, you can get real water temp data which increase beyond 90C often during hard driving.

 

I did some testing few weeks ago.
Our dashboard/system is preprogrammed to show 90 C (or 194F) and not show higher data.

You verified this by examining the software, not by taking water temp readings? What you described would be foolish. Water (without additives) boils at 212F at atmospheric pressure, at higher temps when pressurized. The max temp would be controlled by a thermostat which regulates how much water goes through the radiator(s). That's why it would be pegged at 194F. The dash display thermometer and the thermostat need not be at the same place in the system but if they are or if they use the same data feed then your dash would display exactly what the thermostat is doing. There may be other temp sensors for monitoring fuel, exhaust, oil, etc, which in general will not report exactly the same temp, and may typically be higher.

If you have a problem, either a pressure leak or a malfunctioning thermostat then the temp could go up. A thermostat stuck open results in a car that never warms up. If the temp goes up you really really really want to know about it. That would result in a corresponding increase in pressure until a pressure-release valve, traditionally in the radiator cap, would literally let off steam. When you lose pressure, through a release valve or a leak, the coolant will begin to boil at the hot spots in the engine. When that happens the coolant no longer contacts the metal and that spot gets hotter. It's like oatmeal left unstirred on a stove, eventually it overcooks and you get burned oatmeal. Porsche engines aren't designed to run on oatmeal. The coolant temp display is your advance warning that something is amiss.

Of course there are other ways the dash could inform you of a problem, like the "Check engine" light. But if that's all then why have a temp display at all? I'm usually watching the oil temp on my dash but I imagine that I think that I remember seeing higher than 194F water temp.

 

2019 Cayman S and always been at 194 F. You stated for 2 plus years your water temp has always been 194 F. I suggest you do not ignore this rise in temp.

 

I don't have my new Cayman yet but on the old ones that I did have the water temp went up when I got leaves caught in the vent in the Fall.

 

Common indicator of a failing water pump according to my indy mechanic...

 

That's been my experience with vintage vehicles in the past.