Important ➤ No, #Slack has not been breached again. Company is just sending a quick notification to all those users who have not changed their password since 2015 #databreach and decided to reset passwords for all of them as a precautionary step.

That's probably because someone had successfully cracked passwords that were leaked during the 2015 Slack data breach, even when the company used bcrypt hashing algorithm with a randomly generated salt per-password.