Hundreds of Twitter’s remaining employees have resigned ahead of Elon Musk’s “extremely hardcore” cultural reset of the company, according to internal Slack messages seen by The Verge and employee tweets.

“It feels like all the people who made this place incredible are leaving,” the Twitter staffer said. “It will be extremely hard for Twitter to recover from here, no matter how hardcore the people who remain try to be.”

Multiple “critical” engineering teams inside Twitter have now either completely or near-completely resigned, said another employee who requested anonymity to speak without Musk’s permission. For example, the team that maintains Twitter’s core system libraries that every engineer at the company uses is gone after Thursday. “You cannot run Twitter without this team,” the employee said.

Hundreds of Twitter employees resign after Elon Musk’s ‘hardcore’ ultimatum – The Verge