We should make it possible to disable editing usernames in order to rely on usernames from SSO via a global flag (e.g. DISABLE_USERNAME_EDITS.

Prior art

• Coder Classic has a configurable flag to disable username changes
  
• GitLab and GitHub both allow username changes but preserve the past username to "redirect" links and resources
• GitLab and Sourcegraph have a global configuration to disable username changes