Thanks for asking question! This is excepted behavior. This disablement in the portal is expected as Product group is planning on integrating the different tools/features within Azure portal experience.
Suggest you to refer this blog: Managing an App Service Domain
This will cover common scenarios from Advance Management portal. If there are scenarios that aren’t covered in the blog and you are looking for, Suggest you let us know.
Having said that I am reaching to you internally please respond there if you are looking for something more or any further query on this.