What happens if I put an unsupported version of iOS on my iPhone? I have an iPhone 6s, released in 2015. I want to know if I can install iOS 5.1.1 on it, and if it is possible.

I don't care if it could brick my iPhone. I'm willing to do it.
Simply put: iOS 5 came out in 2011. It does not have the drivers to control hardware from the future.

I guess the real question is: why do you want to do this? Is there some old app that you want to run? Some aspect of the UI that you prefer? Perhaps there's another method of achieving your goal.