Traditional Service Accounts have migrated to Developer Managed Service Accounts (DMSA), a new functionality offering enhanced control and tracking of integration usage.
With DMSA, company administrators can enjoy simplified app management, more secure permissions, improved project access control, and better insights into application usage. As part of this transition Traditional Service Accounts will no longer function. This change phases out older methods of app management replacing it with a more secure and efficient DMSA system for managing your applications. See What is a service Account and Migrating Data Connection Apps to use Developer Managed Service Accounts.