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That's an odd bug. I've spent an hour attempting to replicate it on my Series X with no luck. It seems to allow me to build on any slope no matter how steep it is, as long as it's close enough to water. When not near water it doesn't allow me to build on some steep slopes, but doesn't crash the game either.

If you discover any further steps to replicate the issue, please share it here.
 
That's an odd bug. I've spent an hour attempting to replicate it on my Series X with no luck. It seems to allow me to build on any slope no matter how steep it is, as long as it's close enough to water. When not near water it doesn't allow me to build on some steep slopes, but doesn't crash the game either.

If you discover any further steps to replicate the issue, please share it here.
It just crashed on me just now, I installed the game on internal storage to see if the crash no longer happens, and it still crashes, I was able to reproduce the crash on my Series X by building 3 Fresh Water Outlets and trying to build the 4th Fresh Water Outlet Caused the audio to glitch out briefly before the game crashed. I'm going to attach a video recording of my game crashing.