Some things you just overlook.
The ability to remove custom domains from your Magic Pages site was one of the things I overlooked for a long time.
I always had use cases in mind where customers wanted to change from a .mymagic.page-subdomain to a custom domain, or from one custom domain to another one. But it hadn't occurred to me that sometimes it makes sense to remove a custom domain altogether.
I got the occasional request for this and handled it manually on my end. But given that it's such a simple and straightforward part of managing your website, I have now implemented this as a self-service option in the customer portal.
Removing a custom domain also removes any custom sending domain. Using this option means that you're putting your site back onto the .mymagic.page subdomain.