**EDITED**
In the coming months, there will be a change to remove the custom configuration ARM template file URL and ARM template parameter file URL parameters when creating session hosts. In its place, you will only be able to specify a custom configuration PowerShell script URL that is stored in a publicly accessible location. This change will take effect in the create host pool, add session host to host pool, and getting started portal flows, as well as the GitHub CreateHostpoolTemplate and AddVirtualMachinesToHostPool ARM templates. This is a breaking change, so if you have any dependencies on these parameters, please make the appropriate amends. Note that custom configuration PowerShell scripts are only supported for the operating systems listed here.
I will be sure to reply to this post when the change has been made.
