For context on reasoning, see Discourse topic here: https://community.metabrainz.org/t/814350

Custom domains for Bandcamp are inherently at risk of becoming broken, no longer presenting a Bandcamp release page, as the maintenance of the domain and its DNS records are the duty of the artist/label and not Bandcamp.

I propose a style requirement that URL relationships for releases hosted on Bandcamp use the official domain. The custom one may also be entered, but the official domain would be required.

You can find what the official subdomain is for a custom Bandcamp domain by looking in the HTML <head> element for the <meta> tag containing property="og:url". The content attribute will contain the full path using the official subdomain.

However, note that custom domains for Bandcamp have been broken for a while now, and simply redirect to the official subdomain. You can test the above by using the Internet Archive.