Add button to show/hide subdomains of addon domains
When a customer adds an addon domain to a cPanel account, you have to fill in the subdomain. This is necessary for cPanel to connect the addon domain to an account, which is fine. But this also creates a lot of subdomains in the DNS Zone for the main domain of the account.
There is a long running (4+ years) feature request to remove the requirement for a subdomain: https://features.cpanel.net/topic/remove-requirement-for-a-subdomain-when-creating-an-addon-domain
But since implementing this requires a lot of code refactoring, why not implement a work-around instead which should be quite easy. My suggestion is to add a button or a checkbox (either in WHM or cPanel, or make WHM set the account default and let user override via cPanel) to hide the addon domain's subdomains from the main domain's DNS zone. Just a simple check: if subdomain is connected to addon domain, then hide this row in the table. This should get rid of for example: http://www.addonsubdomain.maindomain.tld in the main domain's DNS zone. If a user modifies the addon domain record, it will internally also modify the connected subdomain. But I think this is already the case.
I think this will solve both this and the existing feature request. One of the first commenters (Sascha) in the existing feature request also briefly mentioned the option for hiding these unnecessary DNS records.
Bonus: also hide these subdomains from dropdown menu's in various places, like the e-mail account creation screen. Just show the addon domains and subdomains (subdomain.addondomain.tld) if they have them, but not (subdomain.)addondomain.maindomain.tld.