How can we improve WHMCS?

This is the default behavior in WHMCS. All services (cancelled and terminated as well) are shown in the dropdown list while creating a support ticket.

When there are many old canceled services or finished domains, the list is really long for the final customer who wants to open a new ticket.

You can say:

"Are shown in the dropdown list while creating a support ticket for cases when a user might want to create a ticket about a cancelled service (to get it reactivated, for example)."

- If the service is canceled or terminated, there is no point in the customer wanting to reactivate it. In this case, you can open a sales ticket requesting it.