Suggestion :

Allow the organisation administrators to configure billing and technical contacts for products owned by their managed accounts.

Workaround : 

• Billing and technical contacts can be changed by other contacts in the product.
• If the sole billing and technical contact already left the company, check with the IT team if it is possible to reactivate the email address of the user to temporarily be used to change the contact for the affected product. 
• If the managed account is an organization admin of the organization that they are billing/technical contacts for, then the organization admin can gain access via discovered products and then navigate to https://admin.atlassian.com/o/<organization ID>/billing/paymentdetails and click "Manage contacts" and remove the managed account from the contacts list.