How can we improve WHMCS?

Maybe this might be solved when users on the Admin-side might specify their own regex for domains. There are currently 3 use cases I identified, where the current decision not to allow subdomains causes problems:

1. Ordering a server or a server-like product:
When as already mentioned in comments, several servers working on the same domain with different hostnames it is
-> hostname1.domainname.tld
or even
-> hostname.location.domain.tld

It is almost never a good idea to have a domain name as a server's hostname. Newbie customers often try this and refrain from this scheme asap.

2. A customer wants to use his own DNS and his/her own domain (no matter if it is hosting, server, service or other).
Those customers usually do know what they do, when they select this option.

3. A customer also might want to use some free domain services like *.de.vu and so on for servers. In this case the limitation WHMCS does not make any sense either.

I can't see any substantial reason why subdomain sequencing should not be allowed, since this really will cause trouble with our support team.

"they were experiencing significant support overhead dealing with customers entering invalid values in this field. It's also generally advisable to validate field data from a security standpoint"

This could be solved by additional validation in the form to make sure invalid values are not accepted and client is promted to correct them.
"Use own subdomain" could also be a sub-option (with proper validation) to the "Use own domain" option.

Hi,

We sell file storage hosting. We have 3k customers who bought these services for the past 1 year and counting. We upgraded to the latest version 2 days ago and this is causing a huge problem for us because we receive at least 10 to 20 orders like that. Now all the orders must happen manually, user has to open a ticket, a staff review the ticket, place the manual order, user has to pay, etc. This is increasing our manpower works, just because WHMCS thinks whats good for us?

I don't think so John, when you remove a feature for majority, then you need to provide a hack for the minority so they won't get affected. I don't think this is smart to remove a feature that existed and hardcode it in the source. The funny part is that you think you know what you are doing but let me be honest with you, you have no idea what your customer needs.

Customer use they own subdomain name to host their files, such as

s1.example.com
s2.example.com
s3.example.com
s4.example.com
s5.example.com

Each subdomain name is a separate account where customer host their files. This way of storing file is good for websites who don't want to host their files on the main hosting as NVME & SSD drives are expensive to be used for storing files from other hand they need the files to be downloaded from the same domain name for branding purposes. Just like Envato's market where all the files are loaded from subdomains.

You may still think that you made the right decision but honestly this added tons of extra work for us, we are paying for automation. Now beside the fact that I could lose customers, I also have to place these order manually.

Please at least provide a hack so we can make it possible for our system. We can't afford this.