Was it a typo on the Website or why is the 2017 10.5 inch iPad Pro (A10X + 4GB of ram) not supported? Because the 7th generation iPad which has an A10 processor + 3GB of ram is supported! It’s weird because the A10X is a more powerful processor so why does it not make the cut?
I have one here (model A1701) and can confirm that the iPadOS 18 beta isn’t available.
The Settings → General → Software Update → Beta Updates menu does now include the iPadOS 18 Developer Beta option, but it has no effect.
I don't have anything to say about the why's, but I can confirm that 2017 iPad Pros are not supported by iOS 18.
2019 iPads (7th generation) are supported by iOS 18.