Former Twitter Security Chief Files Whistleblower Complaints

A whistleblower claims Twitter staff have unsupervised access to sensitive information and accuses management of concealing vulnerabilities. The whistleblower also alleges that Twitter fails to delete user data after account cancellations. Twitter states the whistleblower was dismissed due to poor performance.

What issues does the whistleblower highlight regarding access to sensitive information?

Why did Twitter let go of the whistleblower according to the company's statement?