I run MSFS on an iMac 2017 with a built in Radeon Pro 580 GPU. Actually, I am satisfied with the performance and everything runs really stable in the meantime. However, I am worried that the permanently high temperature of 95 degrees Celsius might damage the GPU over time. In addition, the fan runs at its maximum speed of 2700 rpm while flying.
I’m less bothered by the loud fan noise, but I’m not sure if the continuous load could damage the computer in the long run. I don’t have any other application (neither on MacOS nor under Boot Camp) that puts so much load on the GPU.
Or am I worrying for nothing and the design of the computer ensures that it throttles down in an emergency anyway?
You should definitely worry.
You don’t get throttling when using Windows on a Mac via bootcamp. Throttling is part of the Mac OS software and hardware. You get throttling support when using parallels but at that point you’re not using the full GPU and CPU.
You could buy a bunch of fans and expose the back to keep it cool if you want to use it for MSFS - plus point is it will sound like a a320 cockpit! 
I managed to bring the temperature of the GPU below 90 C through various settings in MSFS (especially reduction to 20 FPS). The fan now also purrs more quietly with less than 2000 revolutions. 20 FPS are stable, there are no dropouts. And otherwise I still have all settings on HIGH-END. I think my iMac will survive longer operating times this way 
And a quiet fan will blow air into the slots from below starting tomorrow and also blow the warm exhaust air at the back. I will see if that brings another 2-3 degrees.
By the way, I have already flown almost 400 hours with the iMac since August 2020.
And probably shortened it’s lifespan by 1600 hours/cycles. Seriously though be careful. The form factor for macs loses cooling efficiency at high temperatures hence its throttling ‘feature’.
If I can lower the temp to 85 C and the fan to 1800 rpm I will be happy. Will continue to experiment.
Well, CPU temperature is now always between 85 and 90 degrees Celsius and fan speed stays below 1800 rpm. So I will have to live with it for the moment.