aws_sso.CfnApplication and custom-saml

[ib353]

I'm trying to create a custom SAML application in IAM Identity Center using the Python CDK, but receive an error:
The application provider with arn 'arn:aws:sso::aws:applicationProvider/app-50e590700beb5208' is not supported for this action. (Service: SsoAdmin, Status Code: 400, Request ID: b2f4f491-368f-432f-ae31-d21bf8c3c49f)

The application provider ARN I'm using is arn:aws:sso::aws:applicationProvider/custom-saml. There appear to be people having similar issues when using Terraform, causing them to contact AWS Support and receive the response that creating custom SAML applications is not supported. This was over a year ago - does anyone know if this is still the case, or has anyone managed to create a custom SAML application through the CDK?

The underlying CloudFormation documentation makes no mention of any issues - would be nice to have the custom-saml string and more details there too.